Earlier quoted context omitted.
JavaScript is forever stuck in design-by-committee hell, which dooms it long-term. Developers who do nothing but pure JavaScript and don't at least play around with the other options (Dart, TypeScript, Haxe, CoffeeScript, et al) are just going to be hurting themselves long-term. It is always a bad idea to tie yourself too strongly to any single language.
Conversely it's a bad idea to spread yourself thin in multiple languages but master of none. Having said that, I'm pretty much tied to Java since 2000 and haven't seen anything wrong... yet. In fact, I'm determined to get better in Java everyday (memory model, JVM, concurrency, better design, etc). Side note: I'm also learning pure "modern" JS just because there's no choice on the client-side.
Knowing Javascript, GWT is a win.