add @MirrorsUsed to observe/polymer/polymer_exprs pkgs

pkg/polymer/lib/src/build/script_compactor.dart

 // Copyright (c) 2013, 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.
 
 /** Transfomer that combines multiple dart script tags into a single one. */
 library polymer.src.build.script_compactor;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
@@ skipping 75 matching lines @@
         document.body.nodes.add(parseFragment(
             '<script src="packages/browser/dart.js"></script>'));
       } else if (dartLoaderTag.parent != document.body) {
         document.body.nodes.add(bootstrapScript);
       } else {
         document.body.insertBefore(bootstrapScript, dartLoaderTag);
       }
 
       var urls = libraries.map((id) => assetUrlFor(id, bootstrapId, logger))
           .where((url) => url != null).toList();
-      var buffer = new StringBuffer()..write(_header);
+      var buffer = new StringBuffer()..writeln(MAIN_HEADER);
       for (int i = 0; i < urls.length; i++) {
         buffer.writeln("import '${urls[i]}' as i$i;");
       }
       buffer..write(_mainPrefix)
           ..writeAll(urls.map((url) => "      '$url',\n"))
           ..write(_mainSuffix);
 
       transform.addOutput(new Asset.fromString(bootstrapId, buffer.toString()));
       transform.addOutput(new Asset.fromString(id, document.outerHtml));
     });
   }
 }
 
-const _header = """
+const MAIN_HEADER = """
 library app_bootstrap;
 
 import 'package:polymer/polymer.dart';
-import 'dart:mirrors' show currentMirrorSystem;
-
+@MirrorsUsed(symbols: 'main', override: 'app_bootstrap')

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

same here, remove override?

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

I'd prefer not to, otherwise we risk breaking mirrors for other people in their
code.

+import 'dart:mirrors' show currentMirrorSystem, MirrorsUsed;
 """;
 
 const _mainPrefix = """
 
 void main() {
   initPolymer([
 """;
 
 // TODO(sigmund): investigate alternative to get the baseUri (dartbug.com/12612)
 const _mainSuffix = """
     ], currentMirrorSystem().isolate.rootLibrary.uri.toString());
 }
 """;