Reduce warnings in polymer:

Reduce warnings in polymer:
 - don't report some warnings that come from parsing comments
 - don't report warnings when crawling the html in the linter (resulted in
   warnings being reported many many times)
 - remove incorrect warning for js onfoo events

BUG=
R=jmesserly@google.com

Committed: https://code.google.com/p/dart/source/detail?r=37365

[Siggi Cherem (dart-lang), 2014-06-14 02:24:58]

https://codereview.chromium.org/334003003/diff/20001/pkg/polymer/lib/src/buil...
File pkg/polymer/lib/src/build/linter.dart (right):

https://codereview.chromium.org/334003003/diff/20001/pkg/polymer/lib/src/buil...
pkg/polymer/lib/src/build/linter.dart:91: logger.warning('couldn\'t find
imported asset "${id.path}" in package'
no reason to make this an error (errors make the build fail)

https://codereview.chromium.org/334003003/diff/20001/pkg/polymer/lib/src/buil...
pkg/polymer/lib/src/build/linter.dart:140: if (extendsTag != null &&
!extendsTag.contains('-')) return extendsTag;
this compensates for allowing the warning above

[Jennifer Messerly, 2014-06-15 20:46:18]

Do we still get editor squiggles via build.dart? If so, LGTM

[Siggi Cherem (dart-lang), 2014-06-16 16:36:17]

On 2014/06/15 20:46:18, John Messerly wrote:
> Do we still get editor squiggles via build.dart? If so, LGTM

Yep, we should. This continues to run the linter on every file, just reports
things once.

[Siggi Cherem (dart-lang), 2014-06-16 17:00:30]

Committed patchset #1 manually as r37365 (presubmit successful).