Using dart:js instead of package:js

Fixes #93 — tested using dart-sdk r28990

Using package:js: (dart2js -- minify)

2.4M	out/web/index.html_bootstrap.dart.js
3.0M	out/web/index.html_bootstrap.dart.precompiled.js

Using dart:js: (dart2js --minify)

344K	out/web/index.html_bootstrap.dart.js
524K	out/web/index.html_bootstrap.dart.precompiled.js

Can't test in r29341 because of other issues.