Implement new flatten specification.

pkg/compiler/lib/src/core_types.dart

Index: pkg/compiler/lib/src/core_types.dart
diff --git a/pkg/compiler/lib/src/core_types.dart b/pkg/compiler/lib/src/core_types.dart
index ba71b3f43de43bb26b60a1f571696174bf9b3b84..a1ec975e53341d3fbb5c8946bbbc8464fc2ba204 100644
--- a/pkg/compiler/lib/src/core_types.dart
+++ b/pkg/compiler/lib/src/core_types.dart
@@ -5,6 +5,7 @@
 library dart2js.type_system;
 
 import 'dart_types.dart';
+import 'elements/elements.dart' show ClassElement;
 
 /// The core types in Dart.
 abstract class CoreTypes {
@@ -45,6 +46,9 @@ abstract class CoreTypes {
   /// [elementType] as its type argument.
   InterfaceType iterableType([DartType elementType = const DynamicType()]);
 
+  /// The `Future` class declaration.
+  ClassElement get futureClass;
+
   /// Returns an instance of the `Future` type defined in 'dart:async' with
   /// [elementType] as its type argument.
   InterfaceType futureType([DartType elementType = const DynamicType()]);