Issue 29393. Consider 'v ??= doSomething()' as usage of 'v'.

pkg/analyzer/lib/src/generated/resolver.dart

 // Copyright (c) 2014, 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.
 
 library analyzer.src.generated.resolver;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_resolution_map.dart';
@@ skipping 3685 matching lines @@
     usedElements.addElement(element);
   }
 
   static bool _isReadIdentifier(SimpleIdentifier node) {
     // not reading at all
     if (!node.inGetterContext()) {
       return false;
     }
     // check if useless reading
     AstNode parent = node.parent;
-    if (parent.parent is ExpressionStatement &&
-        (parent is PrefixExpression ||
-            parent is PostfixExpression ||
-            parent is AssignmentExpression && parent.leftHandSide == node)) {
-      // v++;
-      // ++v;
-      // v += 2;
-      return false;
+    if (parent.parent is ExpressionStatement) {
+      if (parent is PrefixExpression || parent is PostfixExpression) {
+        // v++;
+        // ++v;
+        return false;
+      }
+      if (parent is AssignmentExpression && parent.leftHandSide == node) {
+        // v ??= doSomething();
+        //   vs.
+        // v += 2;
+        TokenType operatorType = parent.operator?.type;
+        return operatorType == TokenType.QUESTION_QUESTION_EQ;
+      }
     }
     // OK
     return true;
   }
 }
 
 /**
  * Instances of the class `HintGenerator` traverse a library's worth of dart code at a time to
  * generate hints over the set of sources.
  *
@@ skipping 7209 matching lines @@
       return null;
     }
     if (identical(node.staticElement, variable)) {
       if (node.inSetterContext()) {
         result = true;
       }
     }
     return null;
   }
 }