Support generators containing classes

[nicolo-ribaudo]

Some browsers (e.g. firefox 50) support classes but not generators.
It would be good it regenerator supported untranspiled classes.

REPL

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This issue was originally reported by @boris-petrov at babel/babel#12692:

Bug Report

Current behavior

• REPL

export default async function () {
  return class extends Base {
    foo() {
      return 1;
    }
  };
}

Expected behavior
Code to be transpiled correctly.

Currently Babel blows up with:

unknown Expression of type "ClassExpression"

Using firefox 35 for targets works. As does firefox 53 and above. Similarly to babel/babel#12690.