Make TreeElementMixin._elements public 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..2cf99641d09549a3b985c489686efe91b0ff4a08 100644
--- a/pkg/compiler/lib/src/resolution/secret_tree_element.dart
+++ b/pkg/compiler/lib/src/resolution/secret_tree_element.dart
@@ -22,8 +22,12 @@ 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
@@ -33,20 +37,20 @@ abstract class TreeElementMixin {
 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;
 }
 
 /**
@@ -55,12 +59,13 @@ abstract class StoredTreeElementMixin implements TreeElementMixin {
  *
  * 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;
 }