Revert "Gracefully handle if loaded dill program has no mainMethod set."
Revert "Gracefully handle if loaded dill program has no mainMethod set."

This reverts commit b24e594ebd56d58546350cff69536750a912ebb9.

I got an email this broke Dart/Flutter integration???

BUG=

Review-Url: https://codereview.chromium.org/2799063002 .

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