Correct `uri` rule in Dart.g
Correct `uri` rule in Dart.g

The spec parser previously assumed that an import or export directive
would use a single-line string. However, multi-line strings like
"""lib.dart""" and raw strings like r'''lib.dart''' are allowed as well,
and also mixed forms like "lib" r'''.dart'''.

This CL changes Dart.g such that those forms are supported.

Change-Id: I40abe1a2f095907803371fc4224bd10bbca38c9f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/208880
Reviewed-by: Lasse R.H. Nielsen <80295e4ef244a07779f93eaa95d582bad8e316a5@google.com>
Commit-Queue: Erik Ernst <251bb01623bfda2cd9151e339a6e783ca432874c@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk