Merge CoreTypes and CoreClasses into CommonElements.

pkg/compiler/lib/src/common/backend_api.dart

Index: pkg/compiler/lib/src/common/backend_api.dart
diff --git a/pkg/compiler/lib/src/common/backend_api.dart b/pkg/compiler/lib/src/common/backend_api.dart
index e0c396cf64498da831e133b639507611c8ca6f1b..78d474ebaf3cc3b5b43f4a804eb66561db0ffa7a 100644
--- a/pkg/compiler/lib/src/common/backend_api.dart
+++ b/pkg/compiler/lib/src/common/backend_api.dart
@@ -189,7 +189,7 @@ abstract class Backend extends Target {
   void registerConstSymbol(String name) {}
 
   ClassElement defaultSuperclass(ClassElement element) {
-    return compiler.coreClasses.objectClass;
+    return compiler.commonElements.objectClass;
   }
 
   bool isInterceptorClass(ClassElement element) => false;