Fixed a build break introduced in r2845.  Added java.io.Serializable as an implemented interface of java.lang.String, but failed to add a serialVersionUID field.  This resulted in a serialVersionUID warning which caused the JdtCompilerTest to fail.

Review by: knorton (desk check)

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@2857 8db76d5a-ed1c-0410-87a9-c151d255dfc7