Add option to only run dwc to get warnings and errors, but not emit any code.

Review URL: https://chromiumcodereview.appspot.com//14746007
Preserve package: imports in web ui generated code

R=jmesserly@google.com

Review URL: https://chromiumcodereview.appspot.com//14761006
fix warnings found by dartanalyzer

R=jmesserly@google.com

Review URL: https://chromiumcodereview.appspot.com//14935006
Keep _root for a deprecation period

R=jmesserly@google.com

Review URL: https://chromiumcodereview.appspot.com//14555008
release 0.4.7+2
merged
merged