introduce web_components pkg for consolidated polyfills

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

[Siggi Cherem (dart-lang), 2014/02/10 20:49:23]

maybe mention the fix for link-rel-stylesheet that now will load things with
HTTP.get until we fix it propertly?

[Jennifer Messerly, 2014/02/10 22:17:11]

my memory on this is already fuzzy :) But I don't think it is actually different
then the previous behavior. The reason a code change was necessary is that
HTMLImports is provided by platform.js, whereas previously we never used it. So
we started going down a code path that was dead code before now. My "fix" sends
us back down the working-but-slow code path.

+  * Polymer polyfills are now consolidated in package:web_components, which is
+    identical to platform.js from http://polymer-project.org. This enables
+    interop with elements built in polymer.js.
+  * New feature: `@ObserveProperty('foo bar.baz') myMethod() {...}` will cause

[Jennifer Messerly, 2014/02/08 01:13:29]

forgot to document these last change.

+    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
     are still used for @published and for polymer-expressions.
     **breaking change**: @initMethod and @CustomTag are only supported on
     public classes/methods.
 
 #### Pub version 0.9.5
   * Improvements on how to handle cross-package HTML imports.
 
 #### Pub version 0.9.4
   * Removes unused dependency on csslib.
@@ skipping 16 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