Warn user if DART_SDK does not match editor SDK

editor/tools/plugins/com.google.dart.tools.ui/src/com/google/dart/tools/ui/DartUIMessages.properties

Index: editor/tools/plugins/com.google.dart.tools.ui/src/com/google/dart/tools/ui/DartUIMessages.properties
diff --git a/editor/tools/plugins/com.google.dart.tools.ui/src/com/google/dart/tools/ui/DartUIMessages.properties b/editor/tools/plugins/com.google.dart.tools.ui/src/com/google/dart/tools/ui/DartUIMessages.properties
index a4b1a77569dec467ecb681782740f945ac2d2983..c4449f13c0510c44e3c9d9ac04e2ee17c3cba0c1 100644
--- a/editor/tools/plugins/com.google.dart.tools.ui/src/com/google/dart/tools/ui/DartUIMessages.properties
+++ b/editor/tools/plugins/com.google.dart.tools.ui/src/com/google/dart/tools/ui/DartUIMessages.properties
@@ -98,6 +98,11 @@ FilteredTypesSelectionDialog_searchJob_taskName=Searching
 FilteredTypeSelectionDialog_showContainerForDuplicatesAction=Show &Container for Duplicates
 FilteredTypeSelectionDialog_titleFormat={0} - {1}
 
+DartSdkInvalid_title=Invalid DART_SDK environment variable
+DartSdkInvalid_doesNotExist=The DART_SDK environment variable references a directory that does not exist:\n\n  {0}\n\nThis will cause problems analyzing source and launching SDK tools.
+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.

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

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?

+DartSdkInvalid_dontShowAgain=In the future, never warn about DART_SDK.
+
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