Fix dart2dart warnings.

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);
   ElementAst.forClassLike(this.ast)
@@ skipping 15 matching lines @@
     return result !== null ?
         result : getFirstNotNullResult((e) => e.getSelector(send));
   }
 
   DartType getType(TypeAnnotation annotation) {
     final result = super.getType(annotation);
     return result !== null ?
         result : getFirstNotNullResult((e) => e.getType(annotation));
   }
 
-  getFirstNotNullResult(f(Element element)) {
+  getFirstNotNullResult(f(TreeElements element)) {
     for (final element in treeElements) {
       final result = f(element);
       if (result !== null) return result;
     }
 
     return null;
   }
 }
 
 class VariableListAst extends ElementAst {
@@ skipping 122 matching lines @@
     };
 
     resolvedElements.forEach((element, treeElements) {
       if (!shouldOutput(element)) return;
 
       var elementAst = new ElementAst(parse(element), treeElements);
       if (element.isField()) {
         final list = (element as VariableElement).variables;
         elementAst = elementAsts.putIfAbsent(
             list, () => new VariableListAst(parse(list)));
-        elementAst.add(element, treeElements);
+        (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);
@@ skipping 129 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);