Extract internal computeResultAsync method

Extract internal computeResultAsync method
to be used by the new completion plugin implementation

R=paulberry@google.com

Committed: https://github.com/dart-lang/sdk/commit/417161413f62fc73444327facd42311963b0f37a

[danrubel, 2015-10-21 14:10:49]

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

[Paul Berry, 2015-10-21 14:41:27]

I'm concerned about this refactor because it gives up runtime type safety.  As
the code stands today, computeResolvedCompilationUnitAsync() creates an
_AnalysisFutureHelper<CompilationUnit>; this causes a
PendingFuture<CompilationUnit> to get created, and on completion, the
implementation of PendingFuture<CompilationUnit> assigns the completed value to
a variable of type CompilationUnit.  So if we accidentally used the wrong type,
we would get a checked-mode error.

With the CL, we create an _AnalysisFutureHelper<dynamic>, so no runtime type
checking is done.

Here's an alternative refactor that would preserve the type safety, at the
expense of giving up a little bit of generality: add `target` and `descriptor`
as final fields of _AnalysisFutureHelper (initialized in its constructor).  Get
rid of the arguments to computeAsync(), and instead move the logic from the
callback new a method inside _AnalysisFutureHelper.  That way
computeResolvedCompilationUnitAsync() looks like this:

...
return new _AnalysisFutureHelper<CompilationUnit>(
    this,
    new LibrarySpecificUnit(librarySource, unitSource),
    RESOLVED_UNIT)
  .computeAsync();

It's only slightly less compact than your version, and it keeps the type safety.

[danrubel, 2015-10-21 16:19:23]

The lack of type safety bothered me as well and I was not sure how to solve it.
Good suggestion. Revised CL. Made AnalysisFutureHelper public so that it can be
consumed by the new completion engine that I'm writing. PTAL.

[Paul Berry, 2015-10-21 16:44:07]

lgtm.  Thanks!

[danrubel, 2015-10-21 17:45:26]

Committed patchset #2 (id:20001) manually as
417161413f62fc73444327facd42311963b0f37a (presubmit successful).