Loader improvements (#6615)

packages/flutter/bin/loader/loader_app.dart

@@ -22,7 +22,9 @@ void main() {
             return new Column(
               children: <Widget>[
                 new Flexible(
-                  child: new Container() // TODO(ianh): replace this with our logo in a Center box
+                  child: new Center(
+                    child: new FlutterLogo(size: 100.0),
+                  ),
                 ),
                 new Flexible(
                   child: new Builder(
@@ -38,21 +40,21 @@ void main() {
                         children.add(new Center(child: new CircularProgressIndicator(value: progressMax > 0 ? progress / progressMax : null)));
                       }
                       return new Block(children: children);
-                    }
+                    },
-                  )
+                  ),
                 ),
                 new Flexible(
                   child: new Block(
-                    padding: new EdgeInsets.symmetric(horizontal: 16.0),
+                    padding: new EdgeInsets.symmetric(horizontal: 24.0),
                     children: <Widget>[ new Text(explanation, textAlign: TextAlign.center) ]
-                  )
                   ),
-              ]
+                ),
+              ],
             );
-          }
+          },
-        )
+        ),
-      )
+      ),
-    )
+    ),
   );
   connectionTimeout = new Timer(const Duration(seconds: 8), () {
     setState(() {
@@ -82,6 +84,17 @@ class LoaderBinding extends WidgetsFlutterBinding {
         });
       }
     );
+    registerStringServiceExtension(
+      name: 'loaderShowExplanation',
+      getter: () => explanation,
+      setter: (String value) {
+        connectionTimeout?.cancel();
+        connectionTimeout = null;
+        setState(() {
+          explanation = value;
+        });
+      }
+    );
     registerNumericServiceExtension(
       name: 'loaderSetProgress',
       getter: () => progress,

packages/flutter_tools/lib/src/hot.dart

@@ -263,6 +263,7 @@ class HotRunner extends ResidentRunner {
       return 3;
     }
     _loaderShowMessage('Connecting...', progress: 0);
+    _loaderShowExplanation('You can use hot reload to update your app on the fly, without restarting it.');
     bool devfsResult = await _updateDevFS(
       progressReporter: (int progress, int max) {
         if (progress % 10 == 0)
@@ -340,6 +341,10 @@ class HotRunner extends ResidentRunner {
     }
   }
 
+  void _loaderShowExplanation(String explanation) {
+    currentView.uiIsolate.flutterLoaderShowExplanation(explanation);
+  }
+
   DevFS _devFS;
 
   Future<Uri> _initDevFS() {

packages/flutter_tools/lib/src/vmservice.dart

@@ -809,6 +809,13 @@ class Isolate extends ServiceObjectOwner {
     }).catchError((dynamic error) => null);
   }
 
+  void flutterLoaderShowExplanation(String explanation) {
+    // Invoke loaderShowExplanation; ignore any returned errors.
+    invokeRpcRaw('ext.flutter.loaderShowExplanation', <String, dynamic> {
+      'value': explanation
+    }).catchError((dynamic error) => null);
+  }
+
   void flutterLoaderSetProgress(double progress) {
     // Invoke loaderSetProgress; ignore any returned errors.
     invokeRpcRaw('ext.flutter.loaderSetProgress', <String, dynamic>{