upgrade dart2js to the latest 1.6.0 dev release

Pushed a new version of the dart2js package up to pub; reference that in our pubspec. Also, upgrade our built-in sdk to 1.6.0 to match the dart2js version.

Removed our test guards around the compiler tests; these pass now. Also, one unrelated change to our logging code in builder.dart.

Fixes #2188, fixes #2187, fixes #2758. @dinhviethoa