Add order to DartType.

sdk/lib/_internal/compiler/implementation/elements/elements.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library elements;
 
 
 import '../tree/tree.dart';
 import '../util/util.dart';
 import '../resolution/resolution.dart';
@@ skipping 477 matching lines @@
   }
 
   static bool isListSupertype(Element element, Compiler compiler) {
     LibraryElement coreLibrary = compiler.coreLibrary;
     return element == coreLibrary.find(const SourceString('Iterable'));
   }
 
   /// A `compareTo` function that places [Element]s in a consistent order based
   /// on the source code order.
   static int compareByPosition(Element a, Element b) {
+    if (identical(a, b)) return 0;
     int r = a.getLibrary().compareTo(b.getLibrary());
     if (r != 0) return r;
     r = a.getCompilationUnit().compareTo(b.getCompilationUnit());
     if (r != 0) return r;
     Token positionA = a.position();
     Token positionB = b.position();
     int offsetA = positionA == null ? -1 : positionA.charOffset;
     int offsetB = positionB == null ? -1 : positionB.charOffset;
     r = offsetA.compareTo(offsetB);
     if (r != 0) return r;
@@ skipping 435 matching lines @@
   int get resolutionState;
   Token get beginToken;
   Token get endToken;
 
   // TODO(kasperl): Try to get rid of these.
   void set annotatedElement(Element value);
   void set resolutionState(int value);
 
   MetadataAnnotation ensureResolved(Compiler compiler);
 }