Introduce AST cloning.

lib/compiler/implementation/dart_backend/backend.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.
 
 class ElementAst {
   final Node ast;
   final TreeElements treeElements;
 
   ElementAst(this.ast, this.treeElements);
+
+  factory ElementAst.clone(ast, treeElements) {
+    final cloner = new CloningVisitor(treeElements);
+    return new ElementAst(cloner.visit(ast), cloner.cloneTreeElements);
+  }
+
   ElementAst.forClassLike(this.ast)
       : this.treeElements = new TreeElementMapping();
 }
 
 class AggregatedTreeElements extends TreeElementMapping {
   final List<TreeElements> treeElements;
 
   AggregatedTreeElements() : treeElements = <TreeElements>[];
 
   Element operator[](Node node) {
@@ skipping 144 matching lines @@
                   new ElementAst.forClassLike(parse(element)));
     };
     newClassElementCallback = (ClassElement classElement) {
       if (!shouldOutput(classElement)) return;
       addClass(classElement);
     };
 
     resolvedElements.forEach((element, treeElements) {
       if (!shouldOutput(element)) return;
 
-      var elementAst = new ElementAst(parse(element), treeElements);
+      var elementAst = new ElementAst.clone(parse(element), treeElements);
       if (element.isField()) {
         final list = (element as VariableElement).variables;
         elementAst = elementAsts.putIfAbsent(
             list, () => new VariableListAst(parse(list)));
         (elementAst as VariableListAst).add(element, treeElements);
         element = list;
       }
 
       if (element.isMember()) {
         ClassElement enclosingClass = element.getEnclosingClass();
         assert(enclosingClass.isClass());
         assert(enclosingClass.isTopLevel());
         assert(shouldOutput(enclosingClass));
         addClass(enclosingClass);
         classMembers[enclosingClass].add(element);
         processElement(element, elementAst);
       } else {
         if (!element.isTopLevel()) {
           compiler.cancel(reason: 'Cannot process $element', element: element);
         }
         addTopLevel(element, elementAst);
       }
     });
 
     // Create all necessary placeholders.
     PlaceholderCollector collector =
-        new PlaceholderCollector(compiler, fixedMemberNames);
+        new PlaceholderCollector(compiler, fixedMemberNames, elementAsts);
     makePlaceholders(element) {
-      collector.collect(element, elementAsts[element].treeElements);
+      collector.collect(element);
       if (element is ClassElement) {
         classMembers[element].forEach(makePlaceholders);
       }
     }
     topLevelElements.forEach(makePlaceholders);
 
     // Create renames.
     Map<Node, String> renames = new Map<Node, String>();
     Map<LibraryElement, String> imports = new Map<LibraryElement, String>();
     renamePlaceholders(
@@ skipping 105 matching lines @@
 }
 
 compareElements(e0, e1) {
   int result = compareBy((e) => e.getLibrary().uri.toString())(e0, e1);
   if (result != 0) return result;
   return compareBy((e) => e.position().charOffset)(e0, e1);
 }
 
 List<Element> sortElements(Collection<Element> elements) =>
     sorted(elements, compareElements);