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

pkg/analyzer/test/generated/non_error_resolver_test.dart

Index: pkg/analyzer/test/generated/non_error_resolver_test.dart
diff --git a/pkg/analyzer/test/generated/non_error_resolver_test.dart b/pkg/analyzer/test/generated/non_error_resolver_test.dart
index 43775da08119b196a4a18cf6f1a11eb728a6a1aa..33b976963652a1765af50daca70d7932486d3fc3 100644
--- a/pkg/analyzer/test/generated/non_error_resolver_test.dart
+++ b/pkg/analyzer/test/generated/non_error_resolver_test.dart
@@ -6,6 +6,7 @@ library engine.non_error_resolver_test;
 
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
+import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/error.dart';
 import 'package:analyzer/src/generated/parser.dart' show ParserErrorCode;
 import 'package:analyzer/src/generated/source_io.dart';
@@ -2307,6 +2308,36 @@ f([String x = '0']) {
     verify([source]);
   }
 
+  void test_invalidAssignment_ifNullAssignment_compatibleType() {
+    AnalysisOptionsImpl options = new AnalysisOptionsImpl();
+    options.enableNullAwareOperators = true;
+    resetWithOptions(options);
+    Source source = addSource('''
+void f(int i) {
+  num n;
+  n ??= i;
+}
+''');
+    resolve(source);
+    assertNoErrors(source);
+    verify([source]);
+  }
+
+  void test_invalidAssignment_ifNullAssignment_sameType() {
+    AnalysisOptionsImpl options = new AnalysisOptionsImpl();
+    options.enableNullAwareOperators = true;
+    resetWithOptions(options);
+    Source source = addSource('''
+void f(int i) {
+  int j;
+  j ??= i;
+}
+''');
+    resolve(source);
+    assertNoErrors(source);
+    verify([source]);
+  }
+
   void test_invalidAssignment_implicitlyImplementFunctionViaCall_1() {
     // 18341
     //