Correct typing of LinkedHashSet.add. Add unit test.

tests/corelib/hash_set_type_check_test.dart

Index: tests/corelib/hash_set_type_check_test.dart
===================================================================
--- tests/corelib/hash_set_type_check_test.dart	(revision 0)
+++ tests/corelib/hash_set_type_check_test.dart	(working copy)
@@ -0,0 +1,39 @@
+// Copyright (c) 2015, 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.
+//
+// VMOptions=--enable_type_checks
+
+// Tests of hash set type checking.
+
+library hash_set_type_check_test;
+import "package:expect/expect.dart";
+import 'dart:collection';
+
+testSet(Set<String> newSet()) {
+  Set<String> s = newSet();
+  Expect.throws(() => s.add(1), (e) => e is Error);
+  Expect.isNull(s.lookup(1));
+}
+
+
+void testIdentitySet(Set create()) {
+  Set<String> s = create();
+  Expect.throws(() => s.add(1), (e) => e is Error);
+  Expect.isNull(s.lookup(1));
+}
+
+
+void main() {
+  testSet(() => new Set<String>());
+  testSet(() => new HashSet<String>());
+  testSet(() => new LinkedHashSet<String>());
+  testIdentitySet(() => new Set<String>.identity());
+  testIdentitySet(() => new HashSet<String>.identity());
+  testIdentitySet(() => new LinkedHashSet<String>.identity());
+  testIdentitySet(() => new HashSet<String>(equals: (x, y) => identical(x, y),
+                                    hashCode: (x) => identityHashCode(x)));
+  testIdentitySet(
+      () => new LinkedHashSet<String>(equals: (x, y) => identical(x, y),
+                              hashCode: (x) => identityHashCode(x)));
+}