Yeah my thoughts. Although I don't know if it impossible - GWT does work of an AST - no reason why other languages couldn't drive it. Perhaps the contenders would be scala (static) and JavaFX markup stuff (which is also static, and designed for UI).
I know its unpopular, but for GUI stuff I prefer statically typed code. It is more verbose, but I find GUI stuff really hard to test (even with friendly tools) and having the compiler do some more work for me I find saves time on silly errors when manually trying stuff out. Yes I type more (or my IDE does really), and I am doing the work for the compiler to some sense, but I find it easier then any other way I have worked for many years doing this.
Its actually ironic (in the alanis sense), I would love a good static typed GUI language, but a dynamic one on the server ;) Maybe my brain is backwards, but I haven't been convinced so far by anyone who has shown me they actually do anything with what they talk about.