Pass platform sdk as uri to frontend server.
Pass platform sdk as uri to frontend server.

This fixes frontend_server.dart.snapshot creation on Windows.

Change-Id: I8dca91ed37ad7c83e951b6fa6c7a34070a35b666
Reviewed-on: https://dart-review.googlesource.com/45060
Commit-Queue: Alexander Aprelev <dfb74c35a357c2059971a4bf7fe26b6a7c18085d@google.com>
Reviewed-by: Siva Annamalai <2e618a8cbd7bb6b3d4409061ae407d692a098546@google.com>

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