Reorder Future.then checks in StrongTypeSystemImpl.
Reorder Future.then checks in StrongTypeSystemImpl.

It is usually slightly faster to check isNotEmpty then to validate
isFutureThen condition.

R=brianwilkerson@google.com, jmesserly@google.com
BUG=

Review URL: https://codereview.chromium.org/2022513002 .

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