C#
That "yield" keyword that Python 3.3 just added? Guess what mainstream language they got the idea from? The lambda syntax ECMAScript 6, C++11, and CoffeeScript use? Guess where that came from? Hint: NOT Java, Python, Ruby, PHP, Perl, or other open-source language of your choice.
Just wait a few years and watch at how many languages add an "async/await" feature.
This is what Google's Dart is intended to do. Dart has its own byte-code. Google has been forthright that the Dart-to-JavaScript compiler is a "temporary" measure until all browsers support Dart in some hypothetical future (that has about a 0% chance of occurring).
The problem with adding any new language is that it requires support from all browsers to be at all useful. TypeScript is *not* a new language, it's just pre-processed JavaScript (like CoffeeScript). So without making a single phone call to another company, Microsoft has created a language that is 100% compatible with all existing browsers.
8 Catfish = 1 Octo-puss