chore(travis): blacklist Dart SDK 1.7.0-dev.2.0

1.7.0-dev.2.0 fixed a bug in 1.7.0-dev.1.0 but it's causing other
failures.  So now we have both versions blacklisted and will be using
1.7.0-dev.0.1 for the dev channel tests.
feat(TestBed): add whitespace handling to compile

Add whitespace handling to the TestBed’s compile function.

Closes #1262. Previously, strings passed with leading or
trailing whitespace would be compiled incorrectly. Now,
valid HTML with arbitrary whitespace in and surrounding it
can be compiled properly.

Closes #1346