Google Closure? I suspect it will continue to evolve/be supported even if things shift quicker than anticipated from today's Angular.js-Directives middleground to Polymer/Web Components.
And whatever Microsoft's building these days on TypeScript. I wouldn't count them out. I expect they'll get about as much traction as Dart. Not yet sure which I'd bet on though. They're both very different from the plain JS suggestions above, or Facebook-style React.js and both used heavily internally on different projects, I think.
An alternative from back in the day, for an "enterprise-supported" framework would perhaps be Sencha's tools. IBM and maybe others, use Dojo. But I'd bet on TypeScript and Closure surviving and evolving more, at this point.
It's just sad that there's still only one, four year old book on Closure. I'm sure the library's evolved since, JS sure has: http://shop.oreilly.com/product/0636920001416.do
But that's kind of the point. Everybody's contributing code that works for them. Consider Netflix's RxJava/RxJS which they took from Microsoft. It's not really integrated in any framework as a pattern, so if you want to use it, you kind of have to abandon or refactor all your other libraries. And all this stems from the relative immaturity of the browser environment to abstract out these widgets. Not to say things will become perfect in the future, but when you've display libraries reimplementing the DOM, something's definitely broken here and could use fixing in the future. Eventually.
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