Move mojo and mojom from mojo/public/dart to mojo/dart/packages

mojo/public/dart/mojo/lib/src/union.dart

-// Copyright 2015 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-part of bindings;
-
-abstract class Union {
-  void encode(Encoder encoder, int offset);
-}
-
-class UnionError {
-}
-
-class UnsetUnionTagError extends UnionError {
-  final curTag;
-  final requestedTag;
-
-  UnsetUnionTagError(this.curTag, this.requestedTag);
-
-  String toString() {
-    return "Tried to read unset union member: {{requestedTag}} "
-      "current member: {{curTag}}.";
-  }
-}