Implement least upper bound.

tests/compiler/dart2js/least_upper_bound_language_test.dart

+// Copyright (c) 2013, 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.
+
+// Test that dart2js produces the expected static type warnings for least upper
+// bound language tests. This ensures that the analyzer and dart2js agrees
+// on these tests.
+
+import 'warnings_checker.dart';
+
+/// Map from test files to a map of their expected status. If the status map is
+/// `null` no warnings must be missing or unexpected, otherwise the status map
+/// can contain a list of line numbers for keys 'missing' and 'unexpected' for
+/// the warnings of each category.
+const Map<String, dynamic> TESTS = const {
+    'language/least_upper_bound_test.dart': null,
+    'language/least_upper_bound_expansive_test.dart': null,
+};
+
+void main() {
+  checkWarnings(TESTS);
+}