fix a warning on pub build
fix a warning on pub build

warning line 11, column 3 of web/index.html: "platform.js" has been renamed to "webcomponents.js". See http://goo.gl/5HPeuP#polymer_46 for details.
  <script src="packages/web_components/platform.js"></script>
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

MiCHiLU/dart-chrome-app,MiCHiLU/dart-chrome-app,MiCHiLU/dart-chrome-app