pkg/shelf: change helper method on Request and Response

pkg/shelf/lib/src/shelf_unmodifiable_map.dart

+// Copyright (c) 2014, 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.
+
+library shelf.shelf_unmodifiable_map;
+
+import 'dart:collection';
+
+// TODO(kevmoo): use UnmodifiableMapView from SDK once 1.4 ships
+import 'package:collection/wrappers.dart' as pc;
+
+/// A simple wrapper over [pc.UnmodifiableMapView] which avoids re-wrapping
+/// itself.
+class ShelfUnmodifiableMap<V> extends pc.UnmodifiableMapView<String, V> {
+  /// If [source] is a [ShelfUnmodifiableMap] with matching [ignoreKeyCase],
+  /// then [source] is returned.
+  ///
+  /// If [source] is `null` it is treated like an empty map.
+  ///
+  /// If [ignoreKeyCase] is `true`, the keys will have case-insensitive access.
+  ///
+  /// [source] is copied to a new [Map] to ensure changes to the paramater value
+  /// after constructions are not reflected.
+  factory ShelfUnmodifiableMap(Map<String, V> source,
+      {bool ignoreKeyCase: false}) {
+    if (source is ShelfUnmodifiableMap<V>) {
+      return source;
+    }
+
+    if (source == null || source.isEmpty) {
+      return const _EmptyShelfUnmodifiableMap();
+    }
+
+    if (ignoreKeyCase) {
+      // TODO(kevmoo) generalize this model with a 'canonical map' to align with
+      // similiar implementation in http pkg [BaseRequest].
+      var map = new LinkedHashMap<String, V>(
+          equals: (key1, key2) => key1.toLowerCase() == key2.toLowerCase(),
+          hashCode: (key) => key.toLowerCase().hashCode);
+
+      map.addAll(source);
+
+      source = map;
+    } else {
+      source = new Map<String, V>.from(source);
+    }
+
+    return new ShelfUnmodifiableMap<V>._(source);
+  }
+
+  ShelfUnmodifiableMap._(Map<String, V> source) : super(source);
+}
+
+/// An const empty implementation of [ShelfUnmodifiableMap].
+class _EmptyShelfUnmodifiableMap<V> extends pc.DelegatingMap<String, V>
+    implements ShelfUnmodifiableMap<V>  {
+  const _EmptyShelfUnmodifiableMap() : super(const {});
+}