Enables codegen support in polymer

pkg/polymer/CHANGELOG.md

 # changelog
 
 This file contains highlights of what changes on each version of the polymer
 package. We will also note important changes to the polyfill packages if they
 impact polymer: custom_element, html_import, observe, shadow_dom,
 and template_binding.
 
 #### Pub version 0.10.0-dev
+  * The output of pub-build no longer uses mirrors. We replace all uses of
+    mirrors with code generation.
   * Interop with polymer-js elements now works.
   * Polymer polyfills are now consolidated in package:web_components, which is
     identical to platform.js from http://polymer-project.org.
   * Breaking change: "noscript" polymer-elements are created by polymer.js, and
     therefore cannot be extended (subtyped) in Dart. They can still be used
     by Dart elements or applications, however.
   * New feature: `@ObserveProperty('foo bar.baz') myMethod() {...}` will cause
     myMethod to be called when "foo" or "bar.baz" changes.
   * Updated for 0.10.0-dev package:observe and package:template_binding changes.
   * Deploy step removes use of mirrors to initialize polymer elements. Mirrors
@@ skipping 25 matching lines @@
 #### Pub version 0.9.2+4
   * fix linter on SVG and MathML tags with XML namespaces
 
 #### Pub version 0.9.2+3
   * fix [15574](https://code.google.com/p/dart/issues/detail?id=15574),
     event bindings in dart2js, by working around issue
     [15573](https://code.google.com/p/dart/issues/detail?id=15573)
 
 #### Pub version 0.9.2+2
   * fix enteredView in dart2js, by using custom_element >= 0.9.1+1