Github #841: Calling .next() on a closed generator shouldn't throw

This fixes #841, but is still a WIP.

I haven't updated one test because I haven't spent the time to figure out what `W` is yet or how I should update this particular test. Test is `TryCatchGenerator.js`. Suggestions welcome.

    1) Feature Tests Yield TryCatchGenerator.js:
       Error: the string "AssertionError: expected '1 [2] [Error: "next" on closed generator] [Error: "next" on closed generator] [Error: "next" on closed generator]' to equal '1 [2]'" was thrown, throw an Error :)
        at Runner.fail (/Users/fitzgen/src/traceur-compiler/node_modules/mocha/lib/runner.js:197:11)
        at Runner.uncaught (/Users/fitzgen/src/traceur-compiler/node_modules/mocha/lib/runner.js:541:8)
        at process.uncaught (/Users/fitzgen/src/traceur-compiler/node_modules/mocha/lib/runner.js:568:10)
        at process.EventEmitter.emit (events.js:95:17)
        at process._fatalException (node.js:272:26)


Aside: is there a CLA for all Mozilla employees, or do I have to sign one myself?