Make mappedBy lazy.

sdk/lib/_internal/compiler/implementation/dart_backend/renamer.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 dart_backend;
 
 Function get _compareNodes =>
     compareBy((n) => n.getBeginToken().charOffset);
 
 typedef String _Renamer(Renamable renamable);
@@ skipping 172 matching lines @@
     // the same identifier. So the top-used local variables in all functions
     // will be renamed first and will all share the same new identifier.
     List<Set<Node>> allSortedLocals = new List<Set<Node>>();
     for (var functionScope in placeholderCollector.functionScopes.values) {
       // Add current sorted local identifiers to the whole sorted list
       // of all local identifiers for all functions.
       List<LocalPlaceholder> currentSortedPlaceholders =
           sorted(functionScope.localPlaceholders,
               compareBy((LocalPlaceholder ph) => -ph.nodes.length));
       List<Set<Node>> currentSortedNodes =
-          currentSortedPlaceholders.mappedBy((ph) => ph.nodes);
+          currentSortedPlaceholders.mappedBy((ph) => ph.nodes).toList();
       // Make room in all sorted locals list for new stuff.
       while (currentSortedNodes.length > allSortedLocals.length) {
         allSortedLocals.add(new Set<Node>());
       }
       for (int i = 0; i < currentSortedNodes.length; i++) {
         allSortedLocals[i].addAll(currentSortedNodes[i]);
       }
     }
 
     // Rename elements, members and locals together based on their usage count,
@@ skipping 145 matching lines @@
     index ~/= firstCharAlphabet.length;
     int length = otherCharsAlphabet.length;
     while (index >= length) {
       resultBuilder.add(otherCharsAlphabet[index % length]);
       index ~/= length;
     }
     resultBuilder.add(otherCharsAlphabet[index]);
     return resultBuilder.toString();
   }
 }