chore(dart/transform): Integrate protoc into gulp build

This change detects if the user has `protoc` available and, if so, uses
it to generate `.pb.dart` files. If not, pre-built files are used
instead.

modules/angular2/pubspec.yaml

@@ -18,6 +18,7 @@ dependencies:
   intl: '^0.12.4'
   logging: '>=0.9.0 <0.12.0'
   observe: '^0.13.1'
+  protobuf: '^0.4.2'
   quiver: '^0.21.4'
   source_span: '^1.0.0'
   stack_trace: '^1.1.1'