Fixes for SDK 0.5.12

R=jmesserly@google.com

Review URL: https://chromiumcodereview.appspot.com//15645006
Update README.md
Apply pull request #525, fixes tests, and fix a pending TODO

R=jmesserly@google.com

Review URL: https://chromiumcodereview.appspot.com//15810005
Fixes to be able to override output folder in build.dart

R=gram@google.com

Review URL: https://chromiumcodereview.appspot.com//16232005
release bug fix (0.4.9+1)
Merge branch 'master' of https://github.com/dart-lang/web-ui into mangling
Merged with latest SDK