Use List instead of Link in the type system.

tests/compiler/dart2js/lookup_member_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';
@@ skipping 26 matching lines @@
       Expect.isNotNull(member);
       DartType memberType = member.type;
       Expect.equals(expectedType, memberType,
           'Wrong member type for $receiverType.$memberName.');
     }
 
     DartType int_ = env['int'];
     DartType String_ = env['String'];
 
     ClassElement A = env.getElement('A');
-    DartType T = A.typeVariables.head;
+    DartType T = A.typeVariables.first;
     DartType A_T = A.thisType;
     expect(A_T, 'foo', T);
 
     DartType A_int = instantiate(A, [int_]);
     expect(A_int, 'foo', int_);
 
     ClassElement B = env.getElement('B');
-    DartType S = B.typeVariables.head;
+    DartType S = B.typeVariables.first;
     DartType B_S = B.thisType;
     expect(B_S, 'foo', instantiate(A, [S]));
     expect(B_S, 'bar', S);
 
     DartType B_int = instantiate(B, [int_]);
     expect(B_int, 'foo', A_int);
     expect(B_int, 'bar', int_);
 
     ClassElement C = env.getElement('C');
-    DartType U = C.typeVariables.head;
+    DartType U = C.typeVariables.first;
     DartType C_U = C.thisType;
     expect(C_U, 'foo', instantiate(A, [String_]));
     expect(C_U, 'bar', String_);
     expect(C_U, 'baz', U);
     expect(C_U, 'boz', instantiate(B, [U]));
 
     DartType C_int = instantiate(C, [int_]);
     expect(C_int, 'foo', instantiate(A, [String_]));
     expect(C_int, 'bar', String_);
     expect(C_int, 'baz', int_);
     expect(C_int, 'boz', instantiate(B, [int_]));
   }));
 }