Remove some 1.7isms.

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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=68833477
Remove an unused constant that is also incompatible with java6.
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=68834592
Change TypeEnumerator to rely on the default behavior of the TreeScanner rather than checking the kind against an EnumSet.
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=68838183
Fix an IAE that's thrown when attempting to contextualize empty nodes.

TreeContext formerly threw an IAE when requests for line and column information returned Diagnostic.NOPOS. No guarantee exists that valid nodes in a given CompilationUnitTree will be assigned position data other than Diagnostic.NOPOS. In practice, our implementation of SourcePosition returns this value for empty nodes (say, an empty modifier list).

This change makes it so that IAE is only thrown when no NodePath exists between a TreeContext's compilation unit and the node provided. Additionally, it modifies the getNodeStartPostion and getNodeEndPosition methods so that they climb the TreePath when looking for position data. This is so that e.g. an empty modifier list may be contextualized to its associated class.
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=68841400
Fix test failures that aries when running under different Java configurations.

TreeDifferTest was relying on the assumption that TreePath.getPath() would find a valid tree path from CompilationUnitTree A to subtree B if that subtree was identical to one found by a traversal of CompilationUnitTree A. This behavior is apparently not guaranteed.

JavaSourcesSubjectFactoryTest makes some assumptions about how many diagnostics will be reported before failure. This CL changes it to only look for the specific diagnostics for which it is testing.
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=70104902