Implement hashCode on objects stored in a set or used as map keys.

dart/sdk/lib/_internal/compiler/implementation/tree/dartstring.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.
 
 part of tree;
 
 /**
  * The [DartString] type represents a Dart string value as a sequence of Unicode
  * Scalar Values.
  * After parsing, any valid [LiteralString] will contain a [DartString]
@@ skipping 25 matching lines @@
     DartString otherString = other;
     if (length != otherString.length) return false;
     Iterator it1 = iterator;
     Iterator it2 = otherString.iterator;
     while (it1.moveNext()) {
       if (!it2.moveNext()) return false;
       if (it1.current != it2.current) return false;
     }
     return true;
   }
+
+  int get hashCode => throw new UnsupportedError('DartString.hashCode');
+
   String toString() => "DartString#${length}:${slowToString()}";
   SourceString get source;
 }
 
 
 /**
  * A [DartString] where the content is represented by an actual [String].
  */
 class LiteralDartString extends DartString {
   final String string;
@@ skipping 170 matching lines @@
         }
         _current = value;
         break;
       default:
         _current = code;
     }
     return true;
   }
 }