I've worked on several large GWT projects, and if compiling to javascript is impacting your productivity, then your development process is not set up right.
In particular, compilation to javascript should not be inside of your basic edit->compile->test loop. Instead, you should be developing in DevMode (formerly known as "HostedMode") where the code runs in a JVM. The step of compiling to javascript should only happen once or twice as your code starts going towards a production environment.
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I've worked on several large GWT projects, and if compiling to javascript is impacting your productivity, then your development process is not set up right.
In particular, compilation to javascript should not be inside of your basic edit->compile->test loop. Instead, you should be developing in DevMode (formerly known as "HostedMode") where the code runs in a JVM. The step of compiling to javascript should only happen once or twice as your code starts going towards a production environment.