Add dart2jsoptions for browser runtimes.
Add dart2jsoptions for browser runtimes.

The `dart2js_options` flag was used in some other codepaths including
"runtime=d8", this change allows us to pass --dart2js-options when using a
browser runtime as well.

Change-Id: I2864f52aff320ab3c2fbfb5a3b4bce34798a1792
Reviewed-on: https://dart-review.googlesource.com/8738
Reviewed-by: Emily Fortuna <38177631bcca88ccece61f6f92cbfa56c19af864@google.com>

dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk