Move dart2js from sdk/lib/_internal/compiler to pkg/compiler

sdk/lib/_internal/compiler/implementation/resolution/secret_tree_element.dart

-// Copyright (c) 2012, 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.
-
-/**
- * Encapsulates the field [TreeElementMixin._element].
- *
- * This library is an implementation detail of dart2js, and should not
- * be imported except by resolution and tree node libraries, or for
- * testing.
- *
- * We have taken great care to ensure AST nodes can be cached between
- * compiler instances.  Part of this requires that we always access
- * resolution results through TreeElements.
- *
- * So please, do not add additional elements to this library, and do
- * not import it.
- */
-library secret_tree_element;
-
-import '../dart2jslib.dart' show invariant, Spannable;
-
-/// Interface for associating
-abstract class TreeElementMixin {
-  Object get _element;
-  void set _element(Object value);
-}
-
-/// Null implementation of [TreeElementMixin] which does not allow association
-/// of elements.
-///
-/// This class is the superclass of all AST nodes.
-abstract class NullTreeElementMixin implements TreeElementMixin, Spannable {
-
-  // Deliberately using [Object] here to thwart code completion.
-  // You're not really supposed to access this field anyways.
-  Object get _element => null;
-  set _element(_) {
-    assert(invariant(this, false,
-        message: "Elements cannot be associated with ${runtimeType}."));
-  }
-}
-
-/// Actual implementation of [TreeElementMixin] which stores the associated
-/// element in the private field [_element].
-///
-/// This class is mixed into the node classes that are actually associated with
-/// elements.
-abstract class StoredTreeElementMixin implements TreeElementMixin {
-  Object _element;
-}
-
-/**
- * Do not call this method directly.  Instead, use an instance of
- * TreeElements.
- *
- * Using [Object] as return type to thwart code completion.
- */
-Object getTreeElement(TreeElementMixin node) => node._element;
-
-/**
- * Do not call this method directly.  Instead, use an instance of
- * TreeElements.
- */
-void setTreeElement(TreeElementMixin node, Object value) {
-  node._element = value;
-}