Teach gclient to run pub in more directories

At gclient sync time, we now run `pub get` for every Dart package except
`workbench`. This patch ensures that we have all the packages we need locally.

DEPS

@@ -306,7 +306,7 @@ hooks = [
                 '--repository-root', '../../../..',
                 '--dart-sdk-directory',
                 '../../../../third_party/dart-sdk/dart-sdk',
-                '--dirs-to-ignore', 'sky/',
+                '--dirs-to-ignore', 'sky/packages/workbench',
     ],
   },
   {

engine/src/flutter/sky/packages/material_design_icons/.gitignore

@@ -0,0 +1,3 @@
+.pub
+packages
+pubspec.lock

engine/src/flutter/sky/packages/sky_engine/.gitignore

@@ -0,0 +1,3 @@
+.pub
+packages
+pubspec.lock

engine/src/flutter/sky/packages/sky_services/.gitignore

@@ -0,0 +1,3 @@
+.pub
+packages
+pubspec.lock

engine/src/flutter/sky/tools/skyx/.gitignore

@@ -1,2 +1,3 @@
+.pub
 packages
-bin/packages
+pubspec.lock