Use List instead of Link in the type system.

tests/compiler/dart2js/least_upper_bound_test.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 subtype_test;
 
 import 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
 import 'type_test_helper.dart';
 import 'package:compiler/implementation/dart_types.dart';
 import "package:compiler/implementation/elements/elements.dart"
        show Element, ClassElement;
 import 'package:compiler/implementation/util/util.dart'
-       show Link;
 
 void main() {
   testInterface1();
   testInterface2();
   testGeneric();
   testMixin();
   testFunction();
   testTypeVariable();
 }
 
@@ skipping 693 matching lines @@
     //     / \   \
     //    V   X   U
     //   /
     //  W
 
     DartType Object_ = env['Object'];
     DartType A = env['A'];
     DartType B = env['B'];
     DartType C = env['C'];
     ClassElement I = env.getElement('I');
-    DartType S = I.typeVariables.head;
-    DartType T = I.typeVariables.tail.head;
-    DartType U = I.typeVariables.tail.tail.head;
-    DartType V = I.typeVariables.tail.tail.tail.head;
-    DartType W = I.typeVariables.tail.tail.tail.tail.head;
-    DartType X = I.typeVariables.tail.tail.tail.tail.tail.head;
+    DartType S = I.typeVariables[0];
+    DartType T = I.typeVariables[1];
+    DartType U = I.typeVariables[2];
+    DartType V = I.typeVariables[3];
+    DartType W = I.typeVariables[4];
+    DartType X = I.typeVariables[5];
 
     checkLub(DartType a, DartType b, DartType expectedLub) {
       DartType lub = env.computeLeastUpperBound(a, b);
       Expect.equals(expectedLub, lub,
           'Unexpected lub($a,$b) = $lub, expected $expectedLub');
     }
 
     checkLub(Object_, Object_, Object_);
     checkLub(Object_, A, Object_);
     checkLub(Object_, B, Object_);
@@ skipping 100 matching lines @@
     checkLub(X, S, Object_);
     checkLub(X, T, T);
     checkLub(X, U, B);
     checkLub(X, V, T);
     checkLub(X, W, T);
     checkLub(X, X, X);
   }));
 }