VM: Fix bug in deferred loading.

tests/language/regress_28278_test.dart

+// Copyright (c) 2017, 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.
+// VMOptions=--load-deferred-eagerly
+// VMOptions=
+
+import "package:expect/expect.dart";
+
+import "regress_28278_lib.dart" deferred as def;
+
+var result = "";
+
+class D {
+  m() async {
+    await def.loadLibrary();
+    result = def.foo(result += "Here");
+  }
+
+}
+
+main(args) async {
+  var d = new D();
+  await d.m();
+  await d.m();
+  Expect.equals("HereHelloHereHello", result);
+}
+