Tweaks for using task model in Editor.

Tweaks for using task model in Editor.

It works in general, bug currently it is buggy and slow.

Notes:

1. Implicit dependencies backfire.
   If the cache size is not big enough and we end up flushing some CU, then recreating it, it looses its LibraryElement.
   We need an explicit dependency on it.

2. If the cache size is not big enough, this still may cause an infinite analysis cycle.

3. We need to implement activating / deactivating cache partitions to change cache size.

4. Probably we need to set LineInfo into ChangeNotice always.

5. There are bugs in type hierarchy.

6. Semantic highlighting takes too long to display.

7. Overlay changes are not used.

R=brianwilkerson@google.com
BUG=

Committed: https://code.google.com/p/dart/source/detail?r=45811

[Brian Wilkerson, 2015-05-14 22:01:28]

LGTM, at least as a temporary work-around

[scheglov, 2015-05-15 15:04:59]

Committed patchset #1 (id:1) manually as 45811 (presubmit successful).