Better error message for missing main()

[jbdeboer]

If I forget a main(), I get the error: It should tell me what file this is in.

Or better yet, don't try to execute main() on files that don't have a main().

Chrome 29.0 (Linux) ERROR Internal error: '/home/deboer/github/angular-dart/__adapter_dart_unittest.dart': Error: line 87 pos 10: identifier 'test_0.main' cannot be resolved test_0.main(); ^