[kernel] Support external procedures without @ExternalName in treeshaker/findNativeName
[kernel] Support external procedures without @ExternalName in treeshaker/findNativeName

Dart allows 'external' functions which are not connected to their body.
Calling such functions produces runtime error, but they should be accepted
at compile time. Test language_2/external_test verifies this behavior.

This CL fixes treeshaker/findNativeName to return 'null' instead of throwing
an error for 'external' functions without @ExternalName annotation (which
is used to specify native name).

Change-Id: I04ec892af29ffe064ece8fddd6f67c7acdca1139
Reviewed-on: https://dart-review.googlesource.com/37122
Reviewed-by: Vyacheslav Egorov <72c81f38df725d8f1832acb9b5bb99563fc2f1b9@google.com>
Commit-Queue: Alexander Markov <239e774fd29abc1267878f57afe1a8a03e0ca887@google.com>

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