diff --git a/examples/flutter_gallery/lib/demo/calculator/logic.dart b/examples/flutter_gallery/lib/demo/calculator/logic.dart
index e73a3fca229..128d273395d 100644
--- a/examples/flutter_gallery/lib/demo/calculator/logic.dart
+++ b/examples/flutter_gallery/lib/demo/calculator/logic.dart
@@ -83,6 +83,8 @@ class OperationToken extends ExpressionToken {
       case Operation.Division:
         return '  \u00F7  ';
     }
+    assert(operation != null);
+    return null;
   }
 }
 
@@ -257,6 +259,8 @@ class CalcExpression {
       case ExpressionState.NumberWithPoint:
       case ExpressionState.Result:
         return appendOperation(Operation.Subtraction);
+      default:
+        return null;
     }
   }
 
diff --git a/examples/flutter_gallery/lib/demo/grid_list_demo.dart b/examples/flutter_gallery/lib/demo/grid_list_demo.dart
index 18a7d77eb12..f6c434ba45c 100644
--- a/examples/flutter_gallery/lib/demo/grid_list_demo.dart
+++ b/examples/flutter_gallery/lib/demo/grid_list_demo.dart
@@ -130,6 +130,8 @@ class GridDemoPhotoItem extends StatelessWidget {
           child: image
         );
     }
+    assert(tileStyle != null);
+    return null;
   }
 }
 
diff --git a/examples/stocks/lib/main.dart b/examples/stocks/lib/main.dart
index 2c542d374b5..5e7b238ae50 100644
--- a/examples/stocks/lib/main.dart
+++ b/examples/stocks/lib/main.dart
@@ -75,6 +75,8 @@ class StocksAppState extends State<StocksApp> {
           accentColor: Colors.redAccent[200]
         );
     }
+    assert(_configuration.stockMode != null);
+    return null;
   }
 
   Route<Null> _getRoute(RouteSettings settings) {
diff --git a/packages/flutter/lib/src/animation/animations.dart b/packages/flutter/lib/src/animation/animations.dart
index 45442315e29..62490a662c4 100644
--- a/packages/flutter/lib/src/animation/animations.dart
+++ b/packages/flutter/lib/src/animation/animations.dart
@@ -287,6 +287,8 @@ class ReverseAnimation extends Animation<double>
       case AnimationStatus.completed: return AnimationStatus.dismissed;
       case AnimationStatus.dismissed: return AnimationStatus.completed;
     }
+    assert(status != null);
+    return null;
   }
 
   @override
diff --git a/packages/flutter/lib/src/cassowary/expression.dart b/packages/flutter/lib/src/cassowary/expression.dart
index 55742aa6e3a..4b36ee1a27d 100644
--- a/packages/flutter/lib/src/cassowary/expression.dart
+++ b/packages/flutter/lib/src/cassowary/expression.dart
@@ -157,7 +157,6 @@ class Expression extends EquationMember {
     if (!m.isConstant) {
       throw new ParserException(
           'The divisor was not a constant expression', <EquationMember>[this, m]);
-      return null;
     }
 
     return this._applyMultiplicand(1.0 / m.value);
diff --git a/packages/flutter/lib/src/material/button.dart b/packages/flutter/lib/src/material/button.dart
index c9687432749..17575228fcf 100644
--- a/packages/flutter/lib/src/material/button.dart
+++ b/packages/flutter/lib/src/material/button.dart
@@ -227,6 +227,8 @@ class _MaterialButtonState extends State<MaterialButton> {
           return Colors.white30;
       }
     }
+    assert(_colorBrightness != null);
+    return null;
   }
 
   void _handleHighlightChanged(bool value) {
diff --git a/packages/flutter/lib/src/material/dialog.dart b/packages/flutter/lib/src/material/dialog.dart
index 2a5e10c85e7..61f1d0eff0c 100644
--- a/packages/flutter/lib/src/material/dialog.dart
+++ b/packages/flutter/lib/src/material/dialog.dart
@@ -57,12 +57,15 @@ class Dialog extends StatelessWidget {
   final List<Widget> actions;
 
   Color _getColor(BuildContext context) {
-    switch (Theme.of(context).brightness) {
+    Brightness brightness = Theme.of(context).brightness;
+    switch (brightness) {
       case Brightness.light:
         return Colors.white;
       case Brightness.dark:
         return Colors.grey[800];
     }
+    assert(brightness != null);
+    return null;
   }
 
   @override
diff --git a/packages/flutter/lib/src/material/drawer_item.dart b/packages/flutter/lib/src/material/drawer_item.dart
index 89631494866..40f32f5ccdd 100644
--- a/packages/flutter/lib/src/material/drawer_item.dart
+++ b/packages/flutter/lib/src/material/drawer_item.dart
@@ -67,6 +67,8 @@ class DrawerItem extends StatelessWidget {
           return Colors.white30;
         return null; // use default icon theme colour unmodified
     }
+    assert(themeData.brightness != null);
+    return null;
   }
 
   TextStyle _getTextStyle(ThemeData themeData) {
diff --git a/packages/flutter/lib/src/material/icon.dart b/packages/flutter/lib/src/material/icon.dart
index a16e4963388..bb63b26499a 100644
--- a/packages/flutter/lib/src/material/icon.dart
+++ b/packages/flutter/lib/src/material/icon.dart
@@ -65,6 +65,8 @@ class Icon extends StatelessWidget {
       case Brightness.light:
         return Colors.black;
     }
+    assert(brightness != null);
+    return null;
   }
 
   Color _getDefaultColor(BuildContext context) {
diff --git a/packages/flutter/lib/src/material/raised_button.dart b/packages/flutter/lib/src/material/raised_button.dart
index df7e24e8a09..4a0a36877bc 100644
--- a/packages/flutter/lib/src/material/raised_button.dart
+++ b/packages/flutter/lib/src/material/raised_button.dart
@@ -98,12 +98,15 @@ class RaisedButton extends StatelessWidget {
     } else {
       if (disabledColor != null)
         return disabledColor;
-      switch (Theme.of(context).brightness) {
+      Brightness brightness = Theme.of(context).brightness;
+      switch (brightness) {
         case Brightness.light:
           return Colors.black12;
         case Brightness.dark:
           return Colors.white12;
       }
+      assert(brightness != null);
+      return null;
     }
   }
 
diff --git a/packages/flutter/lib/src/material/text_selection.dart b/packages/flutter/lib/src/material/text_selection.dart
index 26e80543b5c..fdeae03dbd4 100644
--- a/packages/flutter/lib/src/material/text_selection.dart
+++ b/packages/flutter/lib/src/material/text_selection.dart
@@ -186,4 +186,6 @@ Widget buildTextSelectionHandle(
         child: handle
       );
   }
+  assert(type != null);
+  return null;
 }
diff --git a/packages/flutter/lib/src/painting/box_painter.dart b/packages/flutter/lib/src/painting/box_painter.dart
index 6954c113efb..88dacc9f618 100644
--- a/packages/flutter/lib/src/painting/box_painter.dart
+++ b/packages/flutter/lib/src/painting/box_painter.dart
@@ -1353,6 +1353,8 @@ class BoxDecoration extends Decoration {
         double distance = (position - center).distance;
         return distance <= math.min(size.width, size.height) / 2.0;
     }
+    assert(shape != null);
+    return null;
   }
 
   @override
diff --git a/packages/flutter/lib/src/painting/text_painter.dart b/packages/flutter/lib/src/painting/text_painter.dart
index face24932be..cdcaf82fbf0 100644
--- a/packages/flutter/lib/src/painting/text_painter.dart
+++ b/packages/flutter/lib/src/painting/text_painter.dart
@@ -125,6 +125,8 @@ class TextPainter {
       case TextBaseline.ideographic:
         return _paragraph.ideographicBaseline;
     }
+    assert(baseline != null);
+    return null;
   }
 
   double _lastMinWidth;
@@ -211,6 +213,8 @@ class TextPainter {
             ?? _getOffsetFromUpstream(offset, caretPrototype)
             ?? emptyOffset;
     }
+    assert(position.affinity != null);
+    return null;
   }
 
   /// Returns a list of rects that bound the given selection.
diff --git a/packages/flutter/lib/src/rendering/box.dart b/packages/flutter/lib/src/rendering/box.dart
index 980f19bd802..c1ebd9398da 100644
--- a/packages/flutter/lib/src/rendering/box.dart
+++ b/packages/flutter/lib/src/rendering/box.dart
@@ -1125,7 +1125,6 @@ abstract class RenderBox extends RenderObject {
           'set a size and lay out any children, or, set sizedByParent to true '
           'so that performResize() sizes the render object.'
         );
-        return true;
       }
       return true;
     });
diff --git a/packages/flutter/lib/src/rendering/list.dart b/packages/flutter/lib/src/rendering/list.dart
index 25f51fd04e6..6cba7fd158e 100644
--- a/packages/flutter/lib/src/rendering/list.dart
+++ b/packages/flutter/lib/src/rendering/list.dart
@@ -82,6 +82,8 @@ class RenderList extends RenderVirtualViewport<ListParentData> {
       case Axis.horizontal:
         return padding.horizontal;
     }
+    assert(mainAxis != null);
+    return null;
   }
 
   double get _preferredExtent {
@@ -108,6 +110,8 @@ class RenderList extends RenderVirtualViewport<ListParentData> {
         assert(debugThrowIfNotCheckingIntrinsics());
         return 0.0;
     }
+    assert(mainAxis != null);
+    return null;
   }
 
   @override
diff --git a/packages/flutter/lib/src/rendering/viewport.dart b/packages/flutter/lib/src/rendering/viewport.dart
index b3a7a473ed4..b6a70d01fa7 100644
--- a/packages/flutter/lib/src/rendering/viewport.dart
+++ b/packages/flutter/lib/src/rendering/viewport.dart
@@ -57,6 +57,8 @@ class ViewportDimensions {
       case ViewportAnchor.end:
         return paintOffset + (containerSize - contentSize);
     }
+    assert(anchor != null);
+    return null;
   }
 
   @override
@@ -108,6 +110,8 @@ class RenderViewportBase extends RenderBox {
       case Axis.vertical:
         return offset.dx == 0.0;
     }
+    assert(direction != null);
+    return null;
   }
 
   /// The offset at which to paint the child.
diff --git a/packages/flutter/lib/src/widgets/banner.dart b/packages/flutter/lib/src/widgets/banner.dart
index 55d06fc24ca..5f2dd73f14a 100644
--- a/packages/flutter/lib/src/widgets/banner.dart
+++ b/packages/flutter/lib/src/widgets/banner.dart
@@ -90,6 +90,8 @@ class BannerPainter extends CustomPainter {
       case BannerLocation.topLeft:
         return 0.0;
     }
+    assert(location != null);
+    return null;
   }
 
   double _translationY(double height) {
@@ -101,6 +103,8 @@ class BannerPainter extends CustomPainter {
       case BannerLocation.topLeft:
         return 0.0;
     }
+    assert(location != null);
+    return null;
   }
 
   double get _rotation {
@@ -112,6 +116,8 @@ class BannerPainter extends CustomPainter {
       case BannerLocation.topLeft:
         return -math.PI / 4.0;
     }
+    assert(location != null);
+    return null;
   }
 }
 
diff --git a/packages/flutter/lib/src/widgets/dismissable.dart b/packages/flutter/lib/src/widgets/dismissable.dart
index fad32790849..8133646dd8d 100644
--- a/packages/flutter/lib/src/widgets/dismissable.dart
+++ b/packages/flutter/lib/src/widgets/dismissable.dart
@@ -245,7 +245,6 @@ class _DismissableState extends State<Dismissable> {
           return vy > _kMinFlingVelocity;
       }
     }
-    return false;
   }
 
   void _handleDragEnd(DragEndDetails details) {
diff --git a/packages/flutter/lib/src/widgets/lazy_block.dart b/packages/flutter/lib/src/widgets/lazy_block.dart
index 8b927cc440d..b7acaca8494 100644
--- a/packages/flutter/lib/src/widgets/lazy_block.dart
+++ b/packages/flutter/lib/src/widgets/lazy_block.dart
@@ -731,6 +731,8 @@ class _LazyBlockElement extends RenderObjectElement {
       case Axis.vertical:
         return size.height;
     }
+    assert(widget.mainAxis != null);
+    return null;
   }
 
   Offset _getMainAxisOffsetForSize(Size size) {
@@ -740,6 +742,8 @@ class _LazyBlockElement extends RenderObjectElement {
       case Axis.vertical:
         return new Offset(0.0, size.height);
     }
+    assert(widget.mainAxis != null);
+    return null;
   }
 
   static RenderBox _getNextWithin(_RenderLazyBlock block, RenderBox child) {
diff --git a/packages/flutter/lib/src/widgets/pageable_list.dart b/packages/flutter/lib/src/widgets/pageable_list.dart
index b572554addd..611f60ff746 100644
--- a/packages/flutter/lib/src/widgets/pageable_list.dart
+++ b/packages/flutter/lib/src/widgets/pageable_list.dart
@@ -224,6 +224,8 @@ abstract class PageableState<T extends Pageable> extends ScrollableState<T> {
       case Axis.vertical:
         return box.size.height;
     }
+    assert(config.scrollDirection != null);
+    return null;
   }
 
   @override
@@ -248,6 +250,8 @@ abstract class PageableState<T extends Pageable> extends ScrollableState<T> {
       case ViewportAnchor.end:
         return (_itemCount - scrollIndex - 1) % itemCount;
     }
+    assert(config.scrollAnchor != null);
+    return null;
   }
 
   @override
@@ -324,6 +328,8 @@ abstract class PageableState<T extends Pageable> extends ScrollableState<T> {
       case PageableListFlingBehavior.stopAtNextPage:
         return _flingToAdjacentItem(scrollVelocity);
     }
+    assert(config.itemsSnapAlignment != null);
+    return null;
   }
 
   @override
diff --git a/packages/flutter/lib/src/widgets/scrollable.dart b/packages/flutter/lib/src/widgets/scrollable.dart
index e2f386f8fd5..889a93dd85e 100644
--- a/packages/flutter/lib/src/widgets/scrollable.dart
+++ b/packages/flutter/lib/src/widgets/scrollable.dart
@@ -285,6 +285,8 @@ class ScrollableState<T extends Scrollable> extends State<T> {
       case ViewportAnchor.end:
         return pixelOffset;
     }
+    assert(config.scrollAnchor != null);
+    return null;
   }
 
   /// Convert a scrollOffset value to the number of pixels to which it corresponds.
diff --git a/packages/flutter/lib/src/widgets/scrollable_list.dart b/packages/flutter/lib/src/widgets/scrollable_list.dart
index 28dca198b70..88497d0ef1c 100644
--- a/packages/flutter/lib/src/widgets/scrollable_list.dart
+++ b/packages/flutter/lib/src/widgets/scrollable_list.dart
@@ -239,6 +239,8 @@ class _VirtualListViewport extends VirtualViewport {
         }
         break;
     }
+    assert(mainAxis != null);
+    return null;
   }
 
   @override
diff --git a/packages/flutter/lib/src/widgets/text_selection.dart b/packages/flutter/lib/src/widgets/text_selection.dart
index c08ea39952b..37feefcd8ca 100644
--- a/packages/flutter/lib/src/widgets/text_selection.dart
+++ b/packages/flutter/lib/src/widgets/text_selection.dart
@@ -294,5 +294,7 @@ class _TextSelectionHandleOverlayState extends State<_TextSelectionHandleOverlay
       case TextDirection.rtl:
         return rtlType;
     }
+    assert(endpoint.direction != null);
+    return null;
   }
 }
diff --git a/packages/flutter/lib/src/widgets/virtual_viewport.dart b/packages/flutter/lib/src/widgets/virtual_viewport.dart
index 2342db5aee6..67c9fc11ef4 100644
--- a/packages/flutter/lib/src/widgets/virtual_viewport.dart
+++ b/packages/flutter/lib/src/widgets/virtual_viewport.dart
@@ -70,6 +70,8 @@ abstract class VirtualViewportElement extends RenderObjectElement {
       case ViewportAnchor.end:
         return scrollOffset;
     }
+    assert(renderObject.anchor != null);
+    return null;
   }
 
   /// Returns a two-dimensional representation of the scroll offset, accounting
diff --git a/packages/flutter/test/widget/render_object_widget_test.dart b/packages/flutter/test/widget/render_object_widget_test.dart
index a8228af6306..0b73b995ab7 100644
--- a/packages/flutter/test/widget/render_object_widget_test.dart
+++ b/packages/flutter/test/widget/render_object_widget_test.dart
@@ -23,12 +23,15 @@ class TestOrientedBox extends SingleChildRenderObjectWidget {
   TestOrientedBox({ Key key, Widget child }) : super(key: key, child: child);
 
   Decoration _getDecoration(BuildContext context) {
-    switch (MediaQuery.of(context).orientation) {
+    Orientation orientation = MediaQuery.of(context).orientation;
+    switch (orientation) {
       case Orientation.landscape:
         return new BoxDecoration(backgroundColor: const Color(0xFF00FF00));
       case Orientation.portrait:
         return new BoxDecoration(backgroundColor: const Color(0xFF0000FF));
     }
+    assert(orientation != null);
+    return null;
   }
 
   @override
diff --git a/packages/flutter_tools/lib/src/application_package.dart b/packages/flutter_tools/lib/src/application_package.dart
index dcd55ebd626..ae3d5df2dff 100644
--- a/packages/flutter_tools/lib/src/application_package.dart
+++ b/packages/flutter_tools/lib/src/application_package.dart
@@ -134,6 +134,8 @@ ApplicationPackage getApplicationPackageForPlatform(TargetPlatform platform) {
     case TargetPlatform.linux_x64:
       return null;
   }
+  assert(platform != null);
+  return null;
 }
 
 class ApplicationPackageStore {
@@ -156,5 +158,6 @@ class ApplicationPackageStore {
       case TargetPlatform.linux_x64:
         return null;
     }
+    return null;
   }
 }
diff --git a/packages/flutter_tools/lib/src/build_info.dart b/packages/flutter_tools/lib/src/build_info.dart
index c8d600e2fee..3664066cc37 100644
--- a/packages/flutter_tools/lib/src/build_info.dart
+++ b/packages/flutter_tools/lib/src/build_info.dart
@@ -53,6 +53,7 @@ String getNameForHostPlatform(HostPlatform platform) {
       return 'linux-x64';
   }
   assert(false);
+  return null;
 }
 
 enum TargetPlatform {
@@ -80,6 +81,7 @@ String getNameForTargetPlatform(TargetPlatform platform) {
       return 'linux-x64';
   }
   assert(false);
+  return null;
 }
 
 TargetPlatform getTargetPlatformForName(String platform) {
@@ -97,6 +99,7 @@ TargetPlatform getTargetPlatformForName(String platform) {
     case 'linux-x64':
       return TargetPlatform.linux_x64;
   }
+  assert(platform != null);
   return null;
 }
 
diff --git a/packages/flutter_tools/lib/src/ios/devices.dart b/packages/flutter_tools/lib/src/ios/devices.dart
index 591eb9eb946..23fd55e3c74 100644
--- a/packages/flutter_tools/lib/src/ios/devices.dart
+++ b/packages/flutter_tools/lib/src/ios/devices.dart
@@ -349,7 +349,6 @@ class IOSDevice extends Device {
     } else {
       return false;
     }
-    return false;
   }
 
   @override