Make LinkedHashMap also have a factory constructor and be customizable

pkg/serialization/lib/src/serialization_helpers.dart

Index: pkg/serialization/lib/src/serialization_helpers.dart
diff --git a/pkg/serialization/lib/src/serialization_helpers.dart b/pkg/serialization/lib/src/serialization_helpers.dart
index ec898574730d0cbba7d0ad886adf306d80a6cb88..d9ad747bfa48a4b88211dcfdb8450ec6279433dd 100644
--- a/pkg/serialization/lib/src/serialization_helpers.dart
+++ b/pkg/serialization/lib/src/serialization_helpers.dart
@@ -180,64 +180,3 @@ class _Sentinel {
   final _wrappedObject;
   const _Sentinel(this._wrappedObject);
 }
-
-/**
- * This is used in the implementation of [IdentityMap]. We wrap all the keys
- * in an [_IdentityMapKey] that compares using the identity of the wrapped
- * objects. It also treats equal primitive values as identical
- * to conserve space.
- */
-class _IdentityMapKey {
-  _IdentityMapKey(this._value);
-  var _value;
-
-  /**
-   * Check if an object is primitive to know if we should compare it using
-   * equality or identity. We don't test null/true/false where it's the same.
-   */
-  _isPrimitive(x) => x is String || x is num;
-
-  operator ==(_IdentityMapKey w) =>
-      _isPrimitive(_value) ? _value == w._value : identical(_value, w._value);
-  get hashCode => _value.hashCode;
-  get object => _value;
-}
-
-/**
- * This provides an identity map. We wrap all the objects in
- * an [_IdentityMapKey] that compares using the identity of the
- * wrapped objects. It also treats equal primitive values as identical
- * to conserve space.
- */
-class IdentityMap<K, V> extends LinkedHashMap<K, V> {
-// TODO(alanknight): Replace with a system identity-based map once
-// one is available. Issue 4161.
-// TODO(lrn): Replace with identity map when custom hash maps are introduced
-// (which is soon).
-
-  // Check before wrapping because some methods may call others, e.g. on
-  // dart2js putIfAbsent calls containsKey, so without this we wrap forever.
-  _wrap(Object key) =>
-      (key is _IdentityMapKey) ? key : new _IdentityMapKey(key);
-  _unwrap(_IdentityMapKey wrapper) => wrapper.object;
-
-  Iterable<K> get keys => super.keys.map((x) => _unwrap(x));
-  Iterable<V> get values => super.values;
-
-  void forEach(void f(K key, V value)) {
-    super.forEach((k, v) => f(_unwrap(k), v));
-  }
-
-  V operator [](K key) => super[_wrap(key)];
-
-  void operator []=(K key, V value) {
-      super[_wrap(key)] = value;
-  }
-
-  V putIfAbsent(K key, Function ifAbsent) =>
-      super.putIfAbsent(_wrap(key), ifAbsent);
-
-  bool containsKey(Object key) => super.containsKey(_wrap(key));
-
-  V remove(Object key) => super.remove(_wrap(key));
-}