Implement least upper bound.

tests/compiler/dart2js/type_test_helper.dart

 // Copyright (c) 2013, 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 type_test_helper;
 
 import 'dart:async';
 import "package:expect/expect.dart";
 import '../../../sdk/lib/_internal/compiler/implementation/dart_types.dart';
 import "compiler_helper.dart";
@@ skipping 53 matching lines @@
   }
 
   bool isSubtype(DartType T, DartType S) {
     return compiler.types.isSubtype(T, S);
   }
 
   bool isMoreSpecific(DartType T, DartType S) {
     return compiler.types.isMoreSpecific(T, S);
   }
 
+  DartType computeLeastUpperBound(DartType T, DartType S) {
+    return compiler.types.computeLeastUpperBound(T, S);
+  }
+
   FunctionType functionType(DartType returnType,
                             List<DartType> parameters,
                             {List<DartType> optionalParameter,
                              Map<String,DartType> namedParameters}) {
     Link<DartType> parameterTypes =
         new Link<DartType>.fromList(parameters);
     Link<DartType> optionalParameterTypes = optionalParameter != null
         ? new Link<DartType>.fromList(optionalParameter)
         : const Link<DartType>();
     var namedParameterNames = new LinkBuilder<String>();
     var namedParameterTypes = new LinkBuilder<DartType>();
     if (namedParameters != null) {
       namedParameters.forEach((String name, DartType type) {
         namedParameterNames.addLast(name);
         namedParameterTypes.addLast(type);
       });
     }
-    FunctionType type = new FunctionType(
+    return new FunctionType(
         compiler.functionClass,
         returnType, parameterTypes, optionalParameterTypes,
         namedParameterNames.toLink(), namedParameterTypes.toLink());
   }
 }