Use entities in CommonElements interface.

tests/compiler/dart2js/mock_compiler.dart

Index: tests/compiler/dart2js/mock_compiler.dart
diff --git a/tests/compiler/dart2js/mock_compiler.dart b/tests/compiler/dart2js/mock_compiler.dart
index 938449e7ee39ab1ecfeeec84651923ba2a6fd177..1f74b7701f33310a72acaa5ba904067f14963e70 100644
--- a/tests/compiler/dart2js/mock_compiler.dart
+++ b/tests/compiler/dart2js/mock_compiler.dart
@@ -149,7 +149,8 @@ class MockCompiler extends Compiler {
       // dynamic invocation the ArgumentTypesRegistry eventually iterates over
       // the interfaces of the Object class which would be 'null' if the class
       // wasn't resolved.
-      commonElements.objectClass.ensureResolved(resolution);
+      ClassElement objectClass = commonElements.objectClass;
+      objectClass.ensureResolved(resolution);
     }).then((_) => uri);
   }