Make SdkLibrary.shortName consistently include 'dart:'

Make SdkLibrary.shortName consistently include 'dart:'

The documentation of SdkLibrary.shortName claimed that it didn't
include the 'dart:' prefix; however the implementation did include it.
The implementation of shortName in _MockSdkLibrary class (used only
for unit testing) did not include the 'dart:' prefix, causing some
unit tests to pass even though the functionality was wrong.

This change updates the docs and _MockSdkLibrary to correspond to the
implementation (the 'dart:' prefix is included).  Additionally it
fixes one usage of shortName which was expecting the 'dart:' prefix to
be absent; this was preventing quick fixes from working correctly.

In addition, the Mock SDK is modified to add a reference from
dart:core to dart:async and from dart:async to dart:math (these
dependencies exist in the real SDK).  This avoids the need for some
workarounds in unit tests.

BUG=dartbug.com/20850
R=scheglov@google.com

Committed: https://code.google.com/p/dart/source/detail?r=40000

[Paul Berry, 2014-09-08 20:34:16]

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

[scheglov, 2014-09-08 20:46:28]

LGTM

[Paul Berry, 2014-09-08 21:02:35]

Committed patchset #1 manually as 40000 (presubmit successful).