Warn user if DART_SDK does not match editor SDK

Warn user if DART_SDK does not match editor SDK
* show dialog warning user if DART_SDK does not match
* remove unused method

No longer needed now that DART_SDK references have been removed from all other tools.

[lukechurch, 2013-11-15 03:19:02]

https://codereview.chromium.org/73603002/diff/1/editor/tools/plugins/com.goog...
File
editor/tools/plugins/com.google.dart.tools.ui/src/com/google/dart/tools/ui/DartUIMessages.properties
(right):

https://codereview.chromium.org/73603002/diff/1/editor/tools/plugins/com.goog...
editor/tools/plugins/com.google.dart.tools.ui/src/com/google/dart/tools/ui/DartUIMessages.properties:103:
DartSdkInvalid_notEqual=The DART_SDK environment variable references a different
SDK than Dart Editor:\n\n  DART_SDK : {0}\n  Editor SDK : {1}\n\nThis will cause
problems analyzing source and launching SDK tools.
In the spirit of our error format of:

- What went wrong
- Why is it a problem
- What should the user do

We probably need a little bit more on the 3rd one. Can we do a call out to an
url showing how to fix the issue?

https://codereview.chromium.org/73603002/diff/1/editor/tools/plugins/com.goog...
File
editor/tools/plugins/com.google.dart.tools.ui/src/com/google/dart/tools/ui/DartUIStartup.java
(right):

https://codereview.chromium.org/73603002/diff/1/editor/tools/plugins/com.goog...
editor/tools/plugins/com.google.dart.tools.ui/src/com/google/dart/tools/ui/DartUIStartup.java:175:
Display.getDefault().asyncExec(new Runnable() {
We should instrument the fact that we've discovered an SDK version mismatch, and
report the two versions as metrics, and the full paths as data

[Brian Wilkerson, 2013-11-15 14:58:31]

LGTM

[pquitslund, 2013-11-15 18:30:11]

LGTM.  Modulo Luke's refinements (which I like), bombs away!