Disallow assigning null to non-nullable variables

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

Index: pkg/analyzer/test/src/task/strong/non_null_checker_test.dart
diff --git a/pkg/analyzer/test/src/task/strong/non_null_checker_test.dart b/pkg/analyzer/test/src/task/strong/non_null_checker_test.dart
index 7b9b933973941f16ec1a328330c89e10378b925a..ac056c2a5433a20a7011ff2bba3aca361027e3f7 100644
--- a/pkg/analyzer/test/src/task/strong/non_null_checker_test.dart
+++ b/pkg/analyzer/test/src/task/strong/non_null_checker_test.dart
@@ -45,13 +45,22 @@ main() {
 
   void test_nullableTypes() {
     // By default x can be set to null.
-    checkFile('''int x = null;''');
+    checkFile('int x = null;');
   }
 
-  @failingTest
   void test_nonnullableTypes() {
     // If `int`s are non-nullable, then this code should throw an error.
-    addFile('''int x = /*error:INVALID_ASSIGNMENT*/null;''');
+    addFile('int x;');
+    addFile('int x = /*error:INVALID_ASSIGNMENT*/null;');
+    addFile('int x = 0;');
+    addFile('''
+int x = 0;
+
+main() {
+  x = 1;
+  x = /*error:INVALID_ASSIGNMENT*/null;
+}
+''');
     check(nonnullableTypes: <String>['dart:core,int']);
   }
 }