java.lang.NoClassDefFoundError: com/google/gwt/dev/Compiler

Hi all, When i upgrated my eclipse's google plug-in,when i click the compile button,it says java.lang.NoClassDefFoundError: com/google/gwt/dev/Compiler Exception in thread main but i can run this project in eclipse, anybody knows the reason? My eclipse version :3.5(Java)

Google Map for GWT 's exception

i added the google map to my gwt project,when i run the host browser first time,the page is ok,but when i refresh the browser ,it throws exception like this: [ERROR] sun/plugin2/main/server/IExplorerPlugin java.lang.NoClassDefFoundError: sun/plugin2/main/server/ IExplorerPlugin at