Use `const Symbol` instead of `new Symbol`.
Use `const Symbol` instead of `new Symbol`.

Otherwise minified tests don't work.

Change-Id: I27e9877f0eb539014e4533346e005dc6ef61c317
Reviewed-on: https://dart-review.googlesource.com/4020
Reviewed-by: Florian Loitsch <12d47e7a27fad71f0f62618c5962ce04e125ad18@google.com>

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