Add a reminder not to move bin/traceur-runtime.js.

Fixes #828.
TBR

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/939

Closes #939, closes #828.
+ basic support for Map

* more careful Map constructor

* clear is not calling from constructor
* set returns this
+ lookupIndex that returns index in entries_ by key
* code style
* getOwnHashObject is not IIFE now
+ tests for NaN and positive and negative zeros
- test for creating Map with another map as argument, due to Map is not support @@iterable now

* single check for reentrantly initialising

* 0, -0
Async Functions: Fix implicit return as well as rethrowing

The async function should complete with undefined if there is no
return. It should also lead to a rejected promise if there is an
uncaught exception.

Fixes #933