Fix source maps strong mode runtime failure in dart2js.
Fix source maps strong mode runtime failure in dart2js.

Fix for https://github.com/dart-lang/sdk/issues/33332

Change-Id: I16c464b5dec85ccef55ecbe2e75d67dbf3481927
Reviewed-on: https://dart-review.googlesource.com/58480
Reviewed-by: Stephen Adams <60cb5b73b7e0cb7dcda53302da4fab6ecfc7fa8b@google.com>
Commit-Queue: Emily Fortuna <38177631bcca88ccece61f6f92cbfa56c19af864@google.com>

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