Convert ContextManager to recursively watch analysis roots.

Convert ContextManager to recursively watch analysis roots.

Previously ContextManager watched the contents of each context folder
independently; this was redundant in the case where one context folder
was nested inside another.  Also it led to some race conditions, since
we had to examine the contents of a folder in order to determine
whether there should be a context associated with it, but we couldn't
begin watching those files for that context until after the context
was created; this left a small window of time where a change to the
files might go unnoticed.

Note that a minor functional change is introduced: if "pub list"
indicates that the package resolution for a given folder is dependent
upon a file outside that folder, we no longer watch that file for
changes.  This shouldn't be a problem, since in practice "pub list"
always lists exactly two files as dependencies, and both of those
files are inside the folder (pubspec.yaml and pubspec.lock).

R=brianwilkerson@google.com

Committed: https://github.com/dart-lang/sdk/commit/f5dab3fd2b82d8459a2677c59ccdf474231cb6f7

[Paul Berry, 2015-07-26 22:21:29]

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

[Brian Wilkerson, 2015-07-27 00:35:39]

LGTM

[Paul Berry, 2015-07-27 03:56:47]

Committed patchset #1 (id:1) manually as
f5dab3fd2b82d8459a2677c59ccdf474231cb6f7 (presubmit successful).