Reduce usage of .enclosingElement to get enclosing class.

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 BailoutException {
   final String reason;
 
   const BailoutException(this.reason);
 }
 
@@ skipping 22 matching lines @@
   void processNativeClasses(Enqueuer world,
                             Collection<LibraryElement> libraries) {
   }
 
   /**
    * Adds given class element with its member element to resolved classes
    * collections.
    */
   void addMemberToClass(Element element, ClassElement classElement) {
     // ${element} should have ${classElement} as enclosing.
-    assert(element.enclosingElement == classElement);
+    assert(element.isMember());
     Set<Element> resolvedElementsInClass = resolvedClassMembers.putIfAbsent(
         classElement, () => new Set<Element>());
     resolvedElementsInClass.add(element);
   }
 
   void assembleProgram() {
     resolvedElements.forEach((element, treeElements) {
       unparseValidator.check(element);
     });
 
@@ skipping 13 matching lines @@
     bool shouldOutput(Element element) =>
       element.kind !== ElementKind.VOID &&
       LIBS_TO_IGNORE.indexOf(element.getLibrary()) == -1 &&
       !isDartCoreLib(compiler, element.getLibrary());
 
     try {
       Emitter emitter = new Emitter(compiler);
       resolvedElements.forEach((element, treeElements) {
         if (!shouldOutput(element)) return;
         if (element.isMember()) {
-          var enclosingClass = element.enclosingElement;
+          ClassElement enclosingClass = element.getEnclosingClass();
           assert(enclosingClass.isClass());
           assert(enclosingClass.isTopLevel());
           addMemberToClass(element, enclosingClass);
           return;
         }
         if (!element.isTopLevel()) {
           bailout('Cannot process non top-level $element');
         }
 
         emitter.outputElement(element);
@@ skipping 20 matching lines @@
  */
 bool isDartCoreLib(Compiler compiler, LibraryElement libraryElement) {
   final libraries = compiler.libraries;
   for (final uri in libraries.getKeys()) {
     if (libraryElement === libraries[uri]) {
       if (uri.startsWith('dart:')) return true;
     }
   }
   return false;
 }