Add support for completion of case labels.

Add support for completion of case labels.

This change addresses completions like the following:

  switch (...) {
    foo: case 1:
      ...
    bar: case 2:
      continue ^;
    baz: case 3:
      ...
  }

Completion at the point marked "^" will show "foo", "bar", and "baz"
as possible completions.

R=danrubel@google.com

Committed: https://code.google.com/p/dart/source/detail?r=42799

[Paul Berry, 2015-01-12 18:26:45]

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

[danrubel, 2015-01-12 19:15:13]

lgtm

[Paul Berry, 2015-01-13 03:11:54]

Committed patchset #1 (id:1) manually as 42799 (presubmit successful).