Support use of dill in TypeEnv tests

tests/compiler/dart2js/needs_no_such_method_test.dart

 // Copyright (c) 2016, 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.
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 import 'package:compiler/src/elements/elements.dart' show ClassElement;
 import 'package:compiler/src/elements/names.dart';
 import 'package:compiler/src/universe/call_structure.dart';
@@ skipping 28 matching lines @@
   Future run(List<String> instantiated) async {
     StringBuffer main = new StringBuffer();
     main.write('main() {');
     for (String cls in instantiated) {
       main.write('new $cls();');
     }
     main.write('}');
     testMode = '$instantiated';
 
     var env = await TypeEnvironment.create(CLASSES,
-        mainSource: main.toString(), useMockCompiler: false);
+        mainSource: main.toString(), compileMode: CompileMode.memory);
     foo = new Selector.call(const PublicName('foo'), CallStructure.NO_ARGS);
     bar = new Selector.call(const PublicName('bar'), CallStructure.NO_ARGS);
     baz = new Selector.call(const PublicName('baz'), CallStructure.NO_ARGS);
 
     closedWorld = env.closedWorld;
     superclass = env.getElement('Superclass');
     subclass = env.getElement('Subclass');
     subtype = env.getElement('Subtype');
   }
 
@@ skipping 220 matching lines @@
   check(subtype, ClassQuery.EXACT, foo, true);
   check(subtype, ClassQuery.EXACT, bar, false);
   check(subtype, ClassQuery.EXACT, baz, true);
   check(subtype, ClassQuery.SUBCLASS, foo, true);
   check(subtype, ClassQuery.SUBCLASS, bar, false);
   check(subtype, ClassQuery.SUBCLASS, baz, true);
   check(subtype, ClassQuery.SUBTYPE, foo, true);
   check(subtype, ClassQuery.SUBTYPE, bar, false);
   check(subtype, ClassQuery.SUBTYPE, baz, true);
 }