Replace mappedBy with map (mappedBy will be removed in the future).

Review URL: https://chromiumcodereview.appspot.com//12221034
Fix web-ui to use source_maps spans.

Review URL: https://chromiumcodereview.appspot.com//12210053
Merge branch 'master' into add_css_polyfill

Conflicts:
	lib/src/emitters.dart
	pubspec.yaml