Handle platform access through prefix in dart2dart

tests/language/regress_21998_4_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.
+
+import 'dart:math' as Math;
+import 'package:expect/expect.dart';
+import 'regress_21998_lib1.dart' as lib1;
+import 'regress_21998_lib2.dart';
+import 'regress_21998_lib3.dart';
+
+main() {
+  Expect.equals(4, new C().m());
+}
+
+class C {
+  max(a) => a;
+
+  m() {
+    return max(Math.max(
+               lib3_max(0, lib2_max(1, 2)), lib1.max('a', 'b', 'cd').length));
+  }
+}