Filter only .html files in web, keep those in lib/. This is as a temporary

Filter only .html files in web, keep those in lib/. This is as a temporary
solution for dartbug.com/15038.  Once dartbug.com/14187 is fixed, we can remove
this extra check.

Note: because anything in lib/ is copied under build/packages/foo, I noticed
that the editor will not display any warnings there (they are considered the
same as warning from other packages). So even with this new filter, we still
achive our goal of keeping the build output warning free.

R=jmesserly@google.com

Committed: https://code.google.com/p/dart/source/detail?r=30216

[Jennifer Messerly, 2013-11-13 02:20:11]

lgtm

[Siggi Cherem (dart-lang), 2013-11-13 02:25:22]

Committed patchset #1 manually as r30216 (presubmit successful).