Add a CaseInsensitiveMap class.

lib/src/case_insensitive_map.dart

+// 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.
+
+library http_parser.case_insensitive_map;
+
+import 'package:collection/collection.dart';
+
+/// A map from case-insensitive strings to values.
+///
+/// Much of HTTP is case-insensitive, so this is useful to have pre-defined.
+class CaseInsensitiveMap<V> extends CanonicalizedMap<String, String, V> {
+  CaseInsensitiveMap()
+      : super((key) => key.toLowerCase(), isValidKey: (key) => key != null);
+
+  CaseInsensitiveMap.from(Map<String, V> other)
+      : super.from(other, (key) => key.toLowerCase(),
+            isValidKey: (key) => key != null);
+}