Split BuildSourceClosuresTask into two tasks; remove unnecessary computation.

Split BuildSourceClosuresTask into two tasks; remove unnecessary computation.

This splits BuildSourceClosuresTask into BuildSourceExportClosureTask,
which produces EXPORT_SOURCE_CLOSURE, and
BuildSourceImportExportClosureTask, which produces
IMPORT_EXPORT_SOURCE_CLOSURE.  IMPORT_SOURCE_CLOSURE is no longer
computed, since it was not being used.

This should reduce the amount of analysis work that needs to be rerun
before we can produce code completions, since we will no longer have
so recompute the import/export source closure (which could potentially
need recomputation for a large number of files) until just prior to
ResolveUnitReferencesTask, which in theory doesn't need to run in
order to produce code completions.

R=brianwilkerson@google.com

Committed: https://github.com/dart-lang/sdk/commit/21b65e700bb72965f5314e779edcb0c676841e66

[Paul Berry, 2015-06-29 16:49:17]

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

[Brian Wilkerson, 2015-06-29 16:58:37]

LGTM

[Paul Berry, 2015-06-29 17:13:13]

Committed patchset #1 (id:1) manually as
21b65e700bb72965f5314e779edcb0c676841e66 (presubmit successful).