add @MirrorsUsed to observe/polymer/polymer_exprs pkgs

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.
 
 /**
  * Custom HTML tags, data binding, and templates for building
  * structured, encapsulated, client-side web apps.
  *
  * Polymer.dart, the next evolution of Web UI,
  * is an in-progress Dart port of the
@@ skipping 25 matching lines @@
  *
  * * [Upgrading to Polymer.dart](http://www.dartlang.org/polymer-dart/upgrading-to-polymer-from-web-ui.html):
  * Tips for converting your apps from Web UI to Polymer.dart.
  */
 library polymer;
 
 import 'dart:async';
 import 'dart:collection' show HashMap;
 import 'dart:html';
 import 'dart:js' as js;
+
+@MirrorsUsed(metaTargets:
+    const [Reflectable, ObservableProperty, CustomTag, 'initMethod'],

[Siggi Cherem (dart-lang), 2013/10/15 21:52:18]

ohhh, so dart2js is smart about understanding that:

reflectable = const Reflectable

and preserving also @reflectable as a result?

That's nice!

[Siggi Cherem (dart-lang), 2013/10/15 21:52:18]

About 'initMethod', should this be 'initMethod' or #initMethod, or
_InitMethodAnnotation?

[Jennifer Messerly, 2013/10/15 22:03:29]

yup

[Jennifer Messerly, 2013/10/15 22:03:29]

originally I was having issues with private types. I'll give it another try.

they can't be symbols AFAIK.

+    override: const ['polymer', 'polymer.deserialize'])

[Siggi Cherem (dart-lang), 2013/10/15 21:52:18]

Q: I wonder if the override should just be '*' so that noone has to declare that
Reflectable/CustomTag/initMethod are reflectable.

I guess if you were to use '*' there would be no way to override elsewhere that
you want to keep everything (like when you use the mirrors import without any
annotations)

[Jennifer Messerly, 2013/10/15 22:03:29]

exactly. Building "*" into Polymer would be bad :). We don't know what other
libraries are doing with their reflection so we don't make any claims.

 import 'dart:mirrors';
 
 import 'package:logging/logging.dart' show Logger, Level;
 import 'package:mdv/mdv.dart' as mdv;
 import 'package:mdv/mdv.dart' show NodeBinding;
 import 'package:meta/meta.dart' show deprecated;
 import 'package:observe/observe.dart';
 import 'package:observe/src/microtask.dart';
 import 'package:path/path.dart' as path;
 import 'package:polymer_expressions/polymer_expressions.dart'
     show PolymerExpressions;
 
 import 'deserialize.dart' as deserialize;
 import 'job.dart';
 import 'platform.dart' as platform;
+import 'src/reflected_type.dart';
 
 export 'package:observe/observe.dart';
 export 'package:observe/html.dart';
 export 'package:observe/src/microtask.dart';
 
 part 'src/declaration.dart';
 part 'src/instance.dart';
 part 'src/loader.dart';