I must admit that there are some features that we forgot to include the public documentation that are specifically targeted at contributors.
The first is something that we call contributor SDKs. These allow you to define a GWT SDK that is backed by the gwt-user, gwt-dev-PLAT/gwt-dev-oophm projects in your eclipse workspace. Configure your workspace per
http://code.google.com/p/google-web-toolkit/source/browse/trunk/eclipse/README.txt, do a single ant build to populate the staging directory, and import the gwt-user, gwt-dev-PLAT and gwt-dev-oophm projects. Then add a new GWT SDK (you'll notice that you get an option to create an SDK uses the GWT source projects in your workspace).
The second feature is very basic support for OOPHM. If you are working against an SDK that supports OOPHM, you'll get a checkbox in the GWT tab of the web app launch configuration which selects between OOPHM or normal hosted mode. Lastly, if you are on OSX and you are using an OOPHM-enabled contributor SDK, we filter the SWT jars out of the launch configuration classpath to prevent eclipse from putting -XstartOnFirstThread and breaking OOPHM.
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Miguel