Use an interning table to compare and hash FileBasedSource objects.

Use an interning table to compare and hash FileBasedSource objects.

Speeds up analysis by about 8%.

Note: the interning table uses both the file path and the URI to
distinguish FileBasedSource objects.  This should eliminate the
strange behaviors we would see in the rare case where a single URI
would get associated with multiple file paths (see dartbug.com/22555).

The interning table is never flushed over the lifetime of the
application.  In practice this should not be a problem, because it
only contains one entry per filename/URI, and the total number of
filenames/URIs analyzed during the course of execution should always
be small.

R=brianwilkerson@google.com

Committed: https://code.google.com/p/dart/source/detail?r=44026

[Paul Berry, 2015-02-25 00:25:53]

paulberry@google.com changed reviewers:
+ brianwilkerson@google.com

[Brian Wilkerson, 2015-02-25 01:21:04]

LGTM

[Paul Berry, 2015-02-25 18:11:06]

Committed patchset #1 (id:1) manually as 44026 (presubmit successful).