Add sharedfrontend package

pkg/sharedfrontend/lib/elements.dart

Index: pkg/sharedfrontend/lib/elements.dart
diff --git a/pkg/sharedfrontend/lib/elements.dart b/pkg/sharedfrontend/lib/elements.dart
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+++ b/pkg/sharedfrontend/lib/elements.dart
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+// 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.
+
+/// Shared element model between the dart analyzer and dart2js.
+
+library elements;
+
+/// The interface `Element` defines the behavior common to all of the elements
+/// in the element model. Generally speaking, the element model is a semantic
+/// model of the program that represents things that are declared with a name
+/// and hence can be referenced elsewhere in the code.
+abstract class Element {
+  // TODO(johnniwinther): Semantic difference. Setter names from the analyzer
+  // contain '=' but doesn't in dart2js.
+  // TODO(johnniwinther,paulberry,brianwilkerson): What about privacy? Maybe
+  // name should be the `Name` class in
+  // 'package:compiler/src/elements/names.dart'.
+  /// The name of this element, or `null` if this element does not have a name.
+  String get name;
+
+  /// The library that contains this element. This will be the element itself if
+  /// it is a library element.
+  LibraryElement get library;
+}
+
+/// The interface `LibraryElement` defines the behavior of elements representing
+/// a library.
+abstract class LibraryElement extends Element {
+
+}
+
+/// The interface `ClassElement` defines the behavior of elements that represent
+/// a class.
+abstract class ClassElement extends Element {
+
+}