Safer FileSystemState.hasUri
Safer FileSystemState.hasUri

SourceFactory.forUri2() can return null.

R=1f7641b6b14c52b9163524ab8d9aabff80176f21@google.com

Bug: https://github.com/dart-lang/sdk/issues/33859
Bug: https://github.com/dart-lang/sdk/issues/33233
Change-Id: I34908cd9d04050cc2afcdc716c6bb68d4f177702
Reviewed-on: https://dart-review.googlesource.com/73242
Reviewed-by: Brian Wilkerson <1f7641b6b14c52b9163524ab8d9aabff80176f21@google.com>
Commit-Queue: Konstantin Shcheglov <09e4d7516628963212bf4aace2f97603d2b706e4@google.com>

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