Implement the new '??=' operator in analyzer.

pkg/analyzer/lib/src/generated/error_verifier.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 engine.resolver.error_verifier;
 
 import "dart:math" as math;
 import 'dart:collection';
 
 import 'package:analyzer/src/generated/static_type_analyzer.dart';
@@ skipping 291 matching lines @@
   Object visitAssertStatement(AssertStatement node) {
     _checkForNonBoolExpression(node);
     return super.visitAssertStatement(node);
   }
 
   @override
   Object visitAssignmentExpression(AssignmentExpression node) {
     sc.TokenType operatorType = node.operator.type;
     Expression lhs = node.leftHandSide;
     Expression rhs = node.rightHandSide;
-    if (operatorType == sc.TokenType.EQ) {
+    if (operatorType == sc.TokenType.EQ ||
+        operatorType == sc.TokenType.QUESTION_QUESTION_EQ) {
       _checkForInvalidAssignment(lhs, rhs);
     } else {
       _checkForInvalidCompoundAssignment(node, lhs, rhs);
       _checkForArgumentTypeNotAssignableForArgument(rhs);
     }
     _checkForAssignmentToFinal(lhs);
     return super.visitAssignmentExpression(node);
   }
 
   @override
@@ skipping 5651 matching lines @@
     toCheck.add(type.element);
     // type arguments
     if (type is InterfaceType) {
       InterfaceType interfaceType = type;
       for (DartType typeArgument in interfaceType.typeArguments) {
         _addTypeToCheck(typeArgument);
       }
     }
   }
 }