dart2js-kernel: Emit error trap for unimplemented features
dart2js-kernel: Emit error trap for unimplemented features

The test will fail with an error like the following, rather than proceed with an incorrect null value.

R=efortuna@google.com

Review URL: https://codereview.chromium.org/2540363002 .

TypeError: Cannot read property 'Unhandled ir.FunctionExpression  (dynamic x) → dynamic => x' of null

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