Revert "Make LinkedHashMap also have a factory constructor and be customizable"

pkg/serialization/test/polyfill_identity_map_test.dart

+// Copyright (c) 2012, 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.
+
+/**
+ * Provide a trivial test for identity based hashed collections, which we
+ * provide here until an implementation is available in the regular libraries.
+ */
+// TODO(alanknight): Remove once identity-hashed collections are available.
+// Issue 4161.
+library identity_set_test;
+
+import 'package:unittest/unittest.dart';
+import 'package:serialization/src/serialization_helpers.dart';
+
+class Foo {
+  var x;
+  Foo(this.x);
+  int get hashCode => x.hashCode;
+  bool operator ==(a) => a.x == x;
+}
+
+main() {
+  test('basic', () {
+    var one = new Foo(3);
+    var two = new Foo(3);
+    var map = new Map();
+    var identityMap = new IdentityMap();
+    map[one] = one;
+    map[two] = two;
+    identityMap[one] = one;
+    identityMap[two] = two;
+    expect(map.length, 1);
+    expect(identityMap.length, 2);
+    for (var each in identityMap.values) {
+      expect(each, one);
+    }
+  });
+
+  test('uniquing primitives', () {
+    var map = new IdentityMap();
+    var one = 'one';
+    var two = new String.fromCharCodes(one.codeUnits);
+    map[one] = 1;
+    expect(map[two], 1);
+    expect(map[one], 1);
+  });
+}