Change contract for ChangeSet to reflect AnalysisContext behavior.

Change contract for ChangeSet to reflect AnalysisContext behavior.

The analysis server relies on changedSources having no effect on files
whose contents are already overridden, and we don't want to rely on
undefined behavior.

This shouldn't affect any clients; we are just specifying the behavior
that already existed.

Also, avoid unnecessarily invalidating analysis when changedSources
contains a source that is already overridden.

R=brianwilkerson@google.com

Committed: https://code.google.com/p/dart/source/detail?r=37426

[Brian Wilkerson, 2014-06-17 20:30:03]

LGTM

[Paul Berry, 2014-06-17 21:22:24]

Committed patchset #1 manually as r37426 (presubmit successful).