checking and inference of null coalescing operator, fixes #24564 and fixes #25387

pkg/analyzer/test/src/task/strong/checker_test.dart

Index: pkg/analyzer/test/src/task/strong/checker_test.dart
diff --git a/pkg/analyzer/test/src/task/strong/checker_test.dart b/pkg/analyzer/test/src/task/strong/checker_test.dart
index 46d99db78ac57da85a0d7f6e547a964c795ca233..55b73a8a4282ac48d6070ddf0b8b6ba601f44a9d 100644
--- a/pkg/analyzer/test/src/task/strong/checker_test.dart
+++ b/pkg/analyzer/test/src/task/strong/checker_test.dart
@@ -1493,6 +1493,23 @@ void main() {
        '''
   });
 
+  testChecker('null coalescing operator', {
+    '/main.dart': '''
+          class A {}
+          class C<T> {}
+          main() {
+            A a, b;
+            a ??= new A();
+            b = b ?? new A();
+
+            // downwards inference
+            C<int> c, d;
+            c ??= /*info:INFERRED_TYPE_ALLOCATION*/new C();
+            d = d ?? /*info:INFERRED_TYPE_ALLOCATION*/new C();
+          }
+       '''
+  });
+
   testChecker('compound assignments', {
     '/main.dart': '''
           class A {