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

pkg/analyzer/test/generated/static_type_warning_code_test.dart

Index: pkg/analyzer/test/generated/static_type_warning_code_test.dart
diff --git a/pkg/analyzer/test/generated/static_type_warning_code_test.dart b/pkg/analyzer/test/generated/static_type_warning_code_test.dart
index 019c07eff67ade35090dadeaca0d3567ce30a1ce..02eca6e174b7399266ec1b10e455e0a409126d9a 100644
--- a/pkg/analyzer/test/generated/static_type_warning_code_test.dart
+++ b/pkg/analyzer/test/generated/static_type_warning_code_test.dart
@@ -355,6 +355,21 @@ main() {
     verify([source]);
   }
 
+  void test_invalidAssignment_ifNullAssignment() {
+    AnalysisOptionsImpl options = new AnalysisOptionsImpl();
+    options.enableNullAwareOperators = true;
+    resetWithOptions(options);
+    Source source = addSource('''
+void f(int i) {
+  double d;
+  d ??= i;
+}
+''');
+    resolve(source);
+    assertErrors(source, [StaticTypeWarningCode.INVALID_ASSIGNMENT]);
+    verify([source]);
+  }
+
   void test_invalidAssignment_instanceVariable() {
     Source source = addSource(r'''
 class A {