Fixes #40. ES5 doesn't require quoted/bracketed IdentifierNames that are also Keywords

Per discussions:
- https://github.com/facebook/regenerator/issues/40
- https://github.com/facebook/regenerator/issues/19

Replaces property access by string key name`foo["throw"]` with property access by IdentifierName `foo.throw`

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>