Move TreeElementMixin to avoid out-of-library private overrides

pkg/compiler/lib/src/resolution/secret_tree_element.dart

Index: pkg/compiler/lib/src/resolution/secret_tree_element.dart
diff --git a/pkg/compiler/lib/src/resolution/secret_tree_element.dart b/pkg/compiler/lib/src/resolution/secret_tree_element.dart
index 635ffe77b648865ba6f613b5c760344e2207a472..9b118a65af613e22dac85bf6e4cf7e468aca0848 100644
--- a/pkg/compiler/lib/src/resolution/secret_tree_element.dart
+++ b/pkg/compiler/lib/src/resolution/secret_tree_element.dart
@@ -2,65 +2,11 @@
 // 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 dart2js.resolution.secret_tree_element;
 
-import '../common.dart';
+// The code in this library was moved to ../tree/nodes.dart to satisfy extra
+// restrictions on overriding private members.
+//
+// [See issue for more details](https://github.com/dart-lang/sdk/issues/28809).
 
-/// 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;
-}
+export '../tree/nodes.dart' show getTreeElement, setTreeElement;