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

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

Index: pkg/analyzer/lib/src/generated/element_resolver.dart
diff --git a/pkg/analyzer/lib/src/generated/element_resolver.dart b/pkg/analyzer/lib/src/generated/element_resolver.dart
index afdf3220c262b7b4c08e8043beefc9f1c053a463..94894c0a16ba9bb1676a8daa1c40c9d4015e54d6 100644
--- a/pkg/analyzer/lib/src/generated/element_resolver.dart
+++ b/pkg/analyzer/lib/src/generated/element_resolver.dart
@@ -146,7 +146,8 @@ class ElementResolver extends SimpleAstVisitor<Object> {
   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) {