With the very recent release of ES6 in June 2015, we're already seeing compatibility pre-compilers like Babel enabling ES6 compatibility in browsers not yet supporting some of it's newer features.

I think the best way I can summarize how radical the changes in ES6 are is by comparing it to the features affected in CSS3. CSS3 bought about some radical new changes (animation, keyframes, transitions etc...) and in much the same way ES6 has evolved to make Javascript an incredibly powerful programming language in it's own right (ES6 continues to evolve into a solid OO language with the introduction of more formalized/standard class structures). However; just like CSS3, it took a while for browsers to adopt the newer standards and, for a period, each vendor implemented it in their own way with a vendor prefix.

I foresee a similar situation with ES6. As browsers slowly catch-up and implement the newer standards, tools and libraries will evolve to offer bleeding edge ES6 support in browsers that don't fully support it.

The World of ECMAScript
ECMAScript6 New Features
Coroutines, Generators, Threads, Yield
Webpack - Package Manager
JSPM - Package Manager