Tighten up dependencies in the task model.

Tighten up dependencies in the task model.

- ResolveLibraryTypeNamesTask doesn't need to depend on RESOLVED_UNIT3
  for all sources in the transitive closure of imports; it only needs
  RESOLVED_UNIT3 for the units constituting the current library.

- ResolveVariableReferencesTask doesn't need to depend on
  LIBRARY_ELEMENT6 for all libraries in the transitive closure of
  imports, because it only resolves references to locals and
  parameters, and they can only resolve to elements in the same
  compilation unit; the transitive closure is not needed until
  ResolveUnitReferencesTask.

- ResolveVariableReferencesTask doesn't need LIBRARY_ELEMENT6 of the
  current library, since it only needs the LibraryElement to
  initialize the scope chain; LIBRARY_ELEMENT1 is sufficient.

- ResolveVariableReferencesTask doesn't need RESOLVED_UNIT3 of the
  current library, since it only needs to ensure that the elements for
  locals and parameters have been built; RESOLVED_UNIT1 is sufficient.

This should help reduce the amount of analysis which has to be
performed before code completions can be produced.

R=scheglov@google.com

Committed: https://github.com/dart-lang/sdk/commit/967d4b382063e41c1204976ca5b5e8152eed32ff

[Paul Berry, 2015-06-26 22:02:08]

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

[scheglov, 2015-06-27 01:45:46]

LGTM

[Paul Berry, 2015-06-27 04:56:25]

Committed patchset #1 (id:1) manually as
967d4b382063e41c1204976ca5b5e8152eed32ff (presubmit successful).