Calling .next() on a closed generator shouldn't throw

`next()` on a closed generator should just return `{value: undefined, done: true}` instead of throwing an exception.

Fixes #841

Review URL: https://github.com/google/traceur-compiler/pull/1006

Closes #1006, closes #841.
Catch rejections in WebPageTranscoder.

Remove call to ErrorReporter inside of Loader: all callers of Loader
are responsible for error handling via reject handlers.

Fix latent error codeUnit.address_.

Follow-on fix to issue #1018.

Review URL: https://github.com/google/traceur-compiler/pull/1023

Closes #1023.
Async functions should not be generator functions

Fixes #1009
0.0.43
Implement async arrow functions

Fixes #992
Added Set and updated Map to include iterators

This adds:
- Set
- forEach for Map and Set
- entries, values & keys to Map
- values to Set
Calling g.throw(e) on a closed generator g should throw e, not a new error.
Use assertThrownEquals in the GeneratorThrow.js test.