[gwt-contrib] Ensure that all base URL's are absolut-ify-ed, no matter where we get them from. (issue1066801)

[unnurg]

Reviewers: amitmanjhi,

Description:
Ensure that all base URL's are absolut-ify-ed, no matter where we get
them from.
Also refactor the compute script base code to be easier to read

Please review this at http://gwt-code-reviews.appspot.com/1066801/show

Affected files:
  M dev/core/src/com/google/gwt/core/ext/linker/impl/computeScriptBase.js
  M  
dev/core/test/com/google/gwt/core/ext/linker/impl/SelectionScriptJavaScriptTest.java


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