Disallow assigning null to non-nullable variables

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

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // These tests are for an experimental feature that treats Dart primitive types
 // (int, bool, double, etc.) as non-nullable. This file is not evidence for an
 // intention to officially support non-nullable primitives in Dart (or general
 // NNBD, for that matter) so don't get too crazy about it.
 
 library analyzer.test.src.task.non_null_primitives.checker_test;
@@ skipping 27 matching lines @@
   var languages = new MyList();
   for (int i = 0; i < languages.length; ++i) {
     print(languages[i]);
   }
 }
 ''');
   }
 
   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']);
   }
 }