Add a CaseInsensitiveMap class.

test/case_insensitive_map_test.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.
+
+import 'package:http_parser/http_parser.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test("provides case-insensitive access to the map", () {
+    var map = new CaseInsensitiveMap();
+    map["fOo"] = "bAr";
+    expect(map, containsPair("FoO", "bAr"));
+
+    map["foo"] = "baz";
+    expect(map, containsPair("FOO", "baz"));
+  });
+
+  test("stores the original key cases", () {
+    var map = new CaseInsensitiveMap();
+    map["fOo"] = "bAr";
+    expect(map, equals({"fOo": "bAr"}));
+  });
+
+  test(".from() converts an existing map", () {
+    var map = new CaseInsensitiveMap.from({"fOo": "bAr"});
+    expect(map, containsPair("FoO", "bAr"));
+    expect(map, equals({"fOo": "bAr"}));
+  });
+}