Support use of dill in TypeEnv tests

tests/compiler/dart2js/needs_no_such_method_test.dart

Index: tests/compiler/dart2js/needs_no_such_method_test.dart
diff --git a/tests/compiler/dart2js/needs_no_such_method_test.dart b/tests/compiler/dart2js/needs_no_such_method_test.dart
index 96199a8440cf57992b07c2abac1410dacadd461b..93a005b2d736085697011e16a41fb9cf86bb8ede 100644
--- a/tests/compiler/dart2js/needs_no_such_method_test.dart
+++ b/tests/compiler/dart2js/needs_no_such_method_test.dart
@@ -46,7 +46,7 @@ testClassSets() async {
     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);