Convert HashSet, LinkedHashSet to factory methods and custom implementations.

tests/language/issue10561_test.dart

Index: tests/language/issue10561_test.dart
diff --git a/tests/language/issue10561_test.dart b/tests/language/issue10561_test.dart
index 7ff80be50f5224fc1602dc5a285c80d004d47261..24c308c9573e4fbe400d4821e49b9b0ed23e99f4 100644
--- a/tests/language/issue10561_test.dart
+++ b/tests/language/issue10561_test.dart
@@ -9,9 +9,9 @@ import "package:expect/expect.dart";
 
 import 'dart:collection';
 
-class Foo extends HashSet {
+class Foo extends Expando {
 }
 
 main() {
-  Expect.equals(0, new Foo().length);
+  Expect.isNull(new Foo()[new Object()]);
 }