Extract class runtime functions into own file

See #1007
Extract generator runtime into its own file

See #1007
Move Object assign, is and mixin to polyfill directory

Move these method out of runtime.js

See #1007
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.
respond to review
rebase