Fix handling of union on DictionaryTypeMask.

tests/compiler/dart2js_extra/17856_test.dart

+// Copyright (c) 2014, 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 "package:expect/expect.dart";
+
+// Regression test for Issue 17856.
+
+void main() {
+  var all = { "a": new A(), "b": new B() };
+
+  A a = all["a"];
+  a.load();
+}
+
+class A {
+  Loader _loader = new Loader();
+
+  load() => _loader.loadAll({
+    'a1': {}
+  });
+}
+
+class B {
+  Loader _loader = new Loader();
+
+  load() => _loader.loadAll({
+    'a2': new DateTime.now(),
+  });
+}
+
+class Loader {
+  loadAll(Map assets) {
+    for(String key in assets.keys) {
+      Expect.isTrue(assets[key] is Map);
+    }
+  }
+}