Remove some more cruft for legacy browsers.

https://code.google.com/p/esprima/issues/detail?id=474
Code coverage: ignore known uncovered branches and statements.

https://code.google.com/p/esprima/issues/detail?id=518
Update code coverage report.

https://code.google.com/p/esprima/issues/detail?id=518
Stub to document ES6 official support.

https://code.google.com/p/esprima/issues/detail?id=499
ES6 arrow function.

Based on the previous work by Yusuke Suzuki, Brandon Benvie,
and Marshall Roch.

https://code.google.com/p/esprima/issues/detail?id=517