Remove redundant call to .replace() in url.js

Thanks to @errorx666 for the bug report, #869

Review URL: https://github.com/google/traceur-compiler/pull/871

Closes #871.
Add support for yield yield

Expand the yield expression if needed and transform that into a machine. Now we correctly support `yield (yield expr)`

Review URL: https://github.com/google/traceur-compiler/pull/866

Closes #866.
Add support for yield in if condition

If the condition of an if statement contains a yield we create a state machine for that expression.

Review URL: https://github.com/google/traceur-compiler/pull/867

Closes #867.
Merge branch 'master' into cps-yield-in-switch