[gwt-contrib] Give a better error message when RunAsyncCode.runAsyncCode is passed something

[spoon]

Reviewers: cromwellian,

Description:
Give a better error message when RunAsyncCode.runAsyncCode is passed
something
other than a class literal.

Please review this at http://gwt-code-reviews.appspot.com/161802

Affected files:
  M dev/core/src/com/google/gwt/dev/jjs/impl/ReplaceRunAsyncs.java
  M dev/core/test/com/google/gwt/dev/jjs/JavaAstConstructor.java
  A  
dev/core/test/com/google/gwt/dev/jjs/impl/ReplaceRunAsyncsErrorMessagesTest.java


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