Implement new flatten specification.

tests/compiler/dart2js/type_test_helper.dart

Index: tests/compiler/dart2js/type_test_helper.dart
diff --git a/tests/compiler/dart2js/type_test_helper.dart b/tests/compiler/dart2js/type_test_helper.dart
index 45cd843b60a2f749caf7de9bfd9dc788ce3503f8..f571b36d3d3ddca5e763ad8e16d5d6ffdcade2df 100644
--- a/tests/compiler/dart2js/type_test_helper.dart
+++ b/tests/compiler/dart2js/type_test_helper.dart
@@ -41,7 +41,9 @@ class TypeEnvironment {
     Compiler compiler;
     bool stopAfterTypeInference = mainSource != null;
     if (mainSource == null) {
-      source = 'main() {}\n$source';
+      source = '''import 'dart:async';
+                  main() {}
+                  $source''';
     } else {
       source = '$mainSource\n$source';
     }
@@ -123,6 +125,10 @@ class TypeEnvironment {
     return compiler.types.computeLeastUpperBound(T, S);
   }
 
+  DartType flatten(DartType T) {
+    return compiler.types.flatten(T);
+  }
+
   FunctionType functionType(DartType returnType,
                             List<DartType> parameters,
                             {List<DartType> optionalParameters: