Added library-level comments for polymer and polymer_expressions

pkg/polymer/lib/polymer.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
- * This library exports all of the commonly used functions and types for
- * building UI's.
+ * Custom HTML tags, data binding, and templates for building
+ * structured, encapsulated, client-side web apps.
  *
- * See this article for more information:
- * <http://www.dartlang.org/articles/dart-web-components/>.
+ * Polymer.dart, the next evolution of Web UI,
+ * is an in-progress Dart port of the
+ * [Polymer project](http://www.polymer-project.org/).
+ * Polymer.dart compiles to JavaScript and runs across the modern web.
+ *
+ * To use polymer.dart in your application,
+ * add a dependency in the app's pubspec.yaml file:
+ *
+ *     dependencies:
+ *       polymer: any

[Jennifer Messerly, 2013/09/04 22:09:35]

should we add a suggestion to get the current major release number from
pub.dartlang.org/packages/polymer and use that here instead of "any"?

for example, if starting a new Polymer app now I would want to use

">=0.7.1 <0.8"

as described here: http://pub.dartlang.org/doc/dependencies.html


(we could make this change later too, of course)

[mem, 2013/09/09 18:40:17]

Done.

+ *
+ * Then import the library into your application:
+ *
+ *     import 'package:polymer/polymer.dart';
+ *
+ * ## Other resources
+ *
+ * * Refer to the
+ * [Polymer.dart](http://www.dartlang.org/polymer-dart/)
+ * home page for example code, project status, and
+ * information about how to get started using Polymer.dart in your apps.
+ *
+ * * Check out the source code at the pub repository:
+ * [polymer.dart package](http://pub.dartlang.org/packages/polymer).
+ *
+ * * If you were using Web UI in your apps,
+ * [Upgrading to Polymer.dart]
+ * (http://www.dartlang.org/polymer-dart/upgrading-to-polymer-from-web-ui.html)
+ * contains some tips to help you convert.
  */
 library polymer;
 
 import 'dart:async';
 import 'dart:mirrors';
 
 import 'package:mdv/mdv.dart' as mdv;
 import 'package:observe/src/microtask.dart';
 import 'package:path/path.dart' as path;
 import 'polymer_element.dart' show registerPolymerElement;
@@ skipping 117 matching lines @@
     print("warning: methods marked with @initMethod should take no "
         "arguments, ${method.simpleName} expects some.");
     return;
   }
   obj.invoke(method.simpleName, const []);
 }
 
 class _InitMethodAnnotation {
   const _InitMethodAnnotation();
 }