Use entities in CommonElements interface.

pkg/compiler/lib/src/resolution/resolution.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.
 
 library dart2js.resolution;
 
 import 'dart:collection' show Queue;
 
 import '../common.dart';
 import '../common/names.dart' show Identifiers;
@@ skipping 166 matching lines @@
                 {'modifier': element.asyncMarker});
           }
           if (functionExpression.body.asReturn() != null &&
               element.asyncMarker.isYielding) {
             reporter.reportErrorMessage(
                 asyncModifier,
                 MessageKind.YIELDING_MODIFIER_ON_ARROW_BODY,
                 {'modifier': element.asyncMarker});
           }
         }
+        ClassElement cls;
         switch (element.asyncMarker) {
           case AsyncMarker.ASYNC:
             registry.registerFeature(Feature.ASYNC);
-            commonElements.futureClass.ensureResolved(resolution);
+            cls = commonElements.futureClass;
             break;
           case AsyncMarker.ASYNC_STAR:
             registry.registerFeature(Feature.ASYNC_STAR);
-            commonElements.streamClass.ensureResolved(resolution);
+            cls = commonElements.streamClass;
             break;
           case AsyncMarker.SYNC_STAR:
             registry.registerFeature(Feature.SYNC_STAR);
-            commonElements.iterableClass.ensureResolved(resolution);
+            cls = commonElements.iterableClass;
             break;
         }
+        cls?.ensureResolved(resolution);
       }
     }
   }
 
   bool _isNativeClassOrExtendsNativeClass(ClassElement classElement) {
     assert(classElement != null);
     while (classElement != null) {
       if (target.isNative(classElement)) return true;
       classElement = classElement.superclass;
     }
@@ skipping 323 matching lines @@
    * called by [resolveClass] and [ClassSupertypeResolver].
    */
   void loadSupertypes(BaseClassElementX cls, Spannable from) {
     measure(() {
       if (cls.supertypeLoadState == STATE_DONE) return;
       if (cls.supertypeLoadState == STATE_STARTED) {
         reporter.reportErrorMessage(
             from, MessageKind.CYCLIC_CLASS_HIERARCHY, {'className': cls.name});
         cls.supertypeLoadState = STATE_DONE;
         cls.hasIncompleteHierarchy = true;
-        cls.allSupertypesAndSelf = commonElements
-            .objectClass.allSupertypesAndSelf
+        ClassElement objectClass = commonElements.objectClass;
+        cls.allSupertypesAndSelf = objectClass.allSupertypesAndSelf
             .extendClass(cls.computeType(resolution));
         cls.supertype = cls.allSupertypes.head;
         assert(invariant(from, cls.supertype != null,
             message: 'Missing supertype on cyclic class $cls.'));
         cls.interfaces = const Link<ResolutionDartType>();
         return;
       }
       cls.supertypeLoadState = STATE_STARTED;
       reporter.withCurrentElement(cls, () {
         // TODO(ahe): Cache the node in cls.
@@ skipping 582 matching lines @@
   TreeElements get treeElements {
     assert(invariant(this, _treeElements != null,
         message: "TreeElements have not been computed for $this."));
     return _treeElements;
   }
 
   void reuseElement() {
     _treeElements = null;
   }
 }