fix #29028, understand promoted type parameters

tests/language_strong/type_variable_promotion_test.dart

Index: tests/language_strong/type_variable_promotion_test.dart
diff --git a/tests/language_strong/type_variable_promotion_test.dart b/tests/language_strong/type_variable_promotion_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..6be5df7dcecac24fb547082fe69bed52c5c9371d
--- /dev/null
+++ b/tests/language_strong/type_variable_promotion_test.dart
@@ -0,0 +1,35 @@
+// Copyright (c) 2017, 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.
+
+import 'package:expect/expect.dart';
+
+class A {}
+
+class B extends A {}
+
+class Foo<T extends A> {
+  String foo(T x) {
+    if (x is B) {
+      var list = [x];
+      return list.runtimeType.toString();
+    }
+    return '';
+  }
+
+  List<T> bar(T x) {
+    var tlist = <T>[];
+    if (x is B) {
+      var list = [x];
+      tlist = list;
+    }
+    return tlist;
+  }
+}
+
+main() {
+  var foo = new Foo<B>();
+  var b = new B();
+  Expect.equals(foo.foo(b), 'List<B>');
+  Expect.listEquals(foo.bar(b), [b]);
+}