Don't get value in SingleMapIterator until it is requested.

pkg/analyzer/lib/src/generated/utilities_collection.dart

 // Copyright (c) 2014, 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.
 
 // This code was auto-generated, is not intended to be edited, and is subject to
 // significant change. Please see the README file for more information.
 
 library engine.utilities.collection;
 
 import "dart:math" as math;
@@ skipping 663 matching lines @@
       throw new NoSuchElementException();
     }
     return _currentKey;
   }
 
   @override
   V get value {
     if (_currentKey == null) {
       throw new NoSuchElementException();
     }
+    if (_currentValue == null) {
+      _currentValue = _map[_currentKey];
+    }
     return _currentValue;
   }
 
   @override
   void set value(V newValue) {
     if (_currentKey == null) {
       throw new NoSuchElementException();
     }
     _currentValue = newValue;
     _map[_currentKey] = newValue;
   }
 
   @override
   bool moveNext() {
     if (_keyIterator.moveNext()) {
       _currentKey = _keyIterator.current;
-      _currentValue = _map[_currentKey];
+      _currentValue = null;
       return true;
     } else {
       _currentKey = null;
       return false;
     }
   }
 
   /**
    * Returns a new [SingleMapIterator] instance for the given [Map].
    */
@@ skipping 23 matching lines @@
   /**
    * Map the key to the value.
    *
    * @param key the token being mapped to the value
    * @param value the token to which the key will be mapped
    */
   void put(Token key, Token value) {
     _map[key] = value;
   }
 }