Add debug target to build sourcemap

Review URL: https://github.com/google/traceur-compiler/pull/467

Closes #467.
Fix conflict issue with alpha renaming of this in arrow functions

By doing the alpha renaming in the outer function first we do the renaming of inner arrow functions too.

Fixes #466

Review URL: https://github.com/google/traceur-compiler/pull/468

Closes #468, closes #466.