Link to the widget catalog from the dartdocs (#9743)

Fixes https://github.com/flutter/flutter/issues/7988

packages/flutter/lib/material.dart

@@ -6,9 +6,12 @@
 ///
 /// To use, import `package:flutter/material.dart`.
 ///
-/// See
-/// [material.google.com](https://material.google.com/)
-/// for an introduction to Material Design.
+/// See also:
+///
+///  * [flutter.io/widgets](https://flutter.io/widgets/)
+///    for a catalog of commonly-used Flutter widgets.
+///  * [material.google.com](https://material.google.com/)
+///    for an introduction to Material Design.
 library material;
 
 export 'src/material/about.dart';

packages/flutter/lib/widgets.dart

@@ -5,6 +5,11 @@
 /// The Flutter widgets framework.
 ///
 /// To use, import `package:flutter/widgets.dart`.
+///
+/// See also:
+///
+///  * [flutter.io/widgets](https://flutter.io/widgets/)
+///    for a catalog of commonly-used Flutter widgets.
 library widgets;
 
 export 'package:vector_math/vector_math_64.dart' show Matrix4;