Fix to ensure that external resources are treated as read-only.

editor/tools/plugins/com.google.dart.tools.ui/src/com/google/dart/tools/ui/internal/text/editor/ExternalCompilationUnitEditorInput.java

Index: editor/tools/plugins/com.google.dart.tools.ui/src/com/google/dart/tools/ui/internal/text/editor/ExternalCompilationUnitEditorInput.java
===================================================================
--- editor/tools/plugins/com.google.dart.tools.ui/src/com/google/dart/tools/ui/internal/text/editor/ExternalCompilationUnitEditorInput.java	(revision 8469)
+++ editor/tools/plugins/com.google.dart.tools.ui/src/com/google/dart/tools/ui/internal/text/editor/ExternalCompilationUnitEditorInput.java	(working copy)
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2011, the Dart project authors.
+ * Copyright (c) 2012, the Dart project authors.
  * 
  * Licensed under the Eclipse Public License v1.0 (the "License"); you may not use this file except
  * in compliance with the License. You may obtain a copy of the License at
@@ -69,8 +69,8 @@
    * @return whether this editor input if modifiable
    */
   public boolean isModifiable() {
-    // TODO (danrubel) Investigate implementing or removing this method in future CL
-    return true;
+    //external files should never be treated as modifiable
+    return false;
   }
 
   @Override