How to use com.google.gwt.dev.util.collect.HashMap

Hi
When I want to use com.google.gwt.dev.util.collect.HashMap , I get
this error :
No source code is available for type
com.google.gwt.dev.util.collect.HashMap<K,V>; did you forget to
inherit a required module?
I should inherit which module?

Of course I can use java.util.HashMap but I think
com.google.gwt.dev.util.collect.HashMap is faster!
What should I do.

Thanks.

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