Make TreeElementMixin._elements public to avoid out-of-library private overrides

pkg/compiler/lib/src/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 dart2js.resolution.secret_tree_element;
 
 import '../common.dart';
 
 /// Interface for associating
 abstract class TreeElementMixin {
-  Object get _element;
-  void set _element(Object value);
+  // We would prefer [resolutionSecretTreeElement] to be a private property, but
+  // that is a problem for DDC's separate compilation model where private
+  // overrides are disallowed.
+  // [See](https://github.com/dart-lang/sdk/issues/28809) for more context.
+  Object get resolutionSecretTreeElement;
+  void set resolutionSecretTreeElement(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(_) {
+  Object get resolutionSecretTreeElement => null;
+  set resolutionSecretTreeElement(_) {
     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].
+/// element in the private field [resolutionSecretTreeElement].
 ///
 /// This class is mixed into the node classes that are actually associated with
 /// elements.
 abstract class StoredTreeElementMixin implements TreeElementMixin {
-  Object _element;
+  Object resolutionSecretTreeElement;
 }
 
 /**
  * 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;
+Object getTreeElement(TreeElementMixin node) =>
+    node.resolutionSecretTreeElement;
 
 /**
  * Do not call this method directly.  Instead, use an instance of
  * TreeElements.
  */
 void setTreeElement(TreeElementMixin node, Object value) {
-  node._element = value;
+  node.resolutionSecretTreeElement = value;
 }