Support use of dill in TypeEnv tests

tests/compiler/dart2js/subtypeset_test.dart

 // Copyright (c) 2015, 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.
 
 // Test for iterators on for [SubclassNode].
 
 library subtypeset_test;
 
 import 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
@@ skipping 23 matching lines @@
       class I implements H {}
       """, mainSource: r"""
       main() {
         new A();
         new C();
         new D();
         new E();
         new F();
         new G();
       }
-      """, useMockCompiler: false).then((env) {
+      """, compileMode: CompileMode.memory).then((env) {
         ClosedWorld world = env.closedWorld;
 
         ClassElement A = env.getElement("A");
         ClassElement B = env.getElement("B");
         ClassElement C = env.getElement("C");
         ClassElement D = env.getElement("D");
         ClassElement E = env.getElement("E");
         ClassElement F = env.getElement("F");
         ClassElement G = env.getElement("G");
         ClassElement H = env.getElement("H");
@@ skipping 14 matching lines @@
         checkClass(B, [B, D, E]);
         checkClass(C, [C, E, F, G, H, B, D, I]);
         checkClass(D, [D]);
         checkClass(E, [E]);
         checkClass(F, [F]);
         checkClass(G, [G]);
         checkClass(H, [H, I]);
         checkClass(I, [I]);
       }));
 }