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

pkg/analyzer/lib/src/generated/element_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 engine.resolver.element_resolver;
 
 import 'dart:collection';
 
 import 'ast.dart';
 import 'element.dart';
@@ skipping 128 matching lines @@
     if (function is ConstructorElement) {
       return function.isConst;
     }
     return false;
   }
 
   @override
   Object visitAssignmentExpression(AssignmentExpression node) {
     sc.Token operator = node.operator;
     sc.TokenType operatorType = operator.type;
-    if (operatorType != sc.TokenType.EQ) {
+    if (operatorType != sc.TokenType.EQ &&
+        operatorType != sc.TokenType.QUESTION_QUESTION_EQ) {
       operatorType = _operatorFromCompoundAssignment(operatorType);
       Expression leftHandSide = node.leftHandSide;
       if (leftHandSide != null) {
         String methodName = operatorType.lexeme;
         DartType staticType = _getStaticType(leftHandSide);
         MethodElement staticMethod =
             _lookUpMethod(leftHandSide, staticType, methodName);
         node.staticElement = staticMethod;
         DartType propagatedType = _getPropagatedType(leftHandSide);
         MethodElement propagatedMethod =
@@ skipping 2747 matching lines @@
 
   @override
   Element get staticElement => null;
 
   @override
   accept(AstVisitor visitor) => null;
 
   @override
   void visitChildren(AstVisitor visitor) {}
 }