Don't use linked maps/sets in http internals.

sdk/lib/io/http_headers.dart

 // Copyright (c) 2013, 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.
 
 part of dart.io;
 
 class _HttpHeaders implements HttpHeaders {
   _HttpHeaders(String this.protocolVersion)
-      : _headers = new Map<String, List<String>>();
+      : _headers = new HashMap<String, List<String>>();
 
   List<String> operator[](String name) {
     name = name.toLowerCase();
     return _headers[name];
   }
 
   String value(String name) {
     name = name.toLowerCase();
     List<String> values = _headers[name];
     if (values == null) return null;
@@ skipping 457 matching lines @@
 }
 
 
 class _HeaderValue implements HeaderValue {
   String _value;
   _UnmodifiableMap<String, String> _parameters;
 
   _HeaderValue([String this._value = "", Map<String, String> parameters]) {
     if (parameters != null) {
       _parameters =
-          new _UnmodifiableMap(new Map<String, String>.from(parameters));
+          new _UnmodifiableMap(new HashMap<String, String>.from(parameters));
     }
   }
 
   static _HeaderValue parse(String value,
                             {parameterSeparator: ";",
                              preserveBackslash: false}) {
     // Parse the string.
     var result = new _HeaderValue();
     result._parse(value, parameterSeparator, preserveBackslash);
     return result;
   }
 
   String get value => _value;
 
   void _ensureParameters() {
     if (_parameters == null) {
-      _parameters = new _UnmodifiableMap(new Map<String, String>());
+      _parameters = new _UnmodifiableMap(new HashMap<String, String>());
     }
   }
 
   Map<String, String> get parameters {
     _ensureParameters();
     return _parameters;
   }
 
   String toString() {
     StringBuffer sb = new StringBuffer();
@@ skipping 37 matching lines @@
         throw new HttpException("Failed to parse header value");
       }
       index++;
     }
 
     void maybeExpect(String expected) {
       if (s[index] == expected) index++;
     }
 
     void parseParameters() {
-      var parameters = new Map<String, String>();
+      var parameters = new HashMap<String, String>();
       _parameters = new _UnmodifiableMap(parameters);
 
       String parseParameterName() {
         int start = index;
         while (!done()) {
           if (s[index] == " " || s[index] == "\t" || s[index] == "=") break;
           index++;
         }
         return s.substring(start, index).toLowerCase();
       }
@@ skipping 255 matching lines @@
   void clear() {
     throw new UnsupportedError("Cannot modify an unmodifiable map");
   }
   void forEach(void f(K key, V value)) => _map.forEach(f);
   Iterable<K> get keys => _map.keys;
   Iterable<V> get values => _map.values;
   int get length => _map.length;
   bool get isEmpty => _map.isEmpty;
   bool get isNotEmpty => _map.isNotEmpty;
 }