[gwt-contrib] For synthetic this refs, use params rather than fields while in constructors (issue752801)

[scottb]

Reviewers: tobyr,



Please review this at http://gwt-code-reviews.appspot.com/752801/show

Affected files:
  M dev/core/src/com/google/gwt/dev/jjs/ast/JField.java
  M dev/core/src/com/google/gwt/dev/jjs/impl/BuildTypeMap.java
  M dev/core/src/com/google/gwt/dev/jjs/impl/GenerateJavaAST.java


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