Refactor sourcemap generation to combine generator and sourceUrl.

This helps ensure the generator has the same sourceURL as the //# source.
Update SourceMap.js test.

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/1561

Closes #1561.
Add Math polyfill

This implements the new ES6 Math functions.

The implementation is based on V8 and MDN as well as the spec.

The tests are mostly just smoke tests. I'm not testing a lot of
edge cases.