Make the parameters map of the HTTP HeaderValue unmodifiable

tests/standalone/io/http_headers_test.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.
 
 import "package:expect/expect.dart";
 import 'dart:async';
 import 'dart:math';
 import 'dart:collection';
 
 part '../../../sdk/lib/io/common.dart';
@@ skipping 233 matching lines @@
     } else {
       Expect.equals(0, contentType.parameters.length);
     }
   }
 
   ContentType contentType;
   contentType = new ContentType("", "");
   Expect.equals("", contentType.primaryType);
   Expect.equals("", contentType.subType);
   Expect.equals("/", contentType.value);
+  Expect.throws(() => contentType.parameters["xxx"] = "yyy",
+                (e) => e is UnsupportedError);
 
   contentType = ContentType.parse("text/html");
   check(contentType, "text", "html");
   Expect.equals("text/html", contentType.toString());
   contentType = new ContentType("text", "html", charset: "utf-8");
   check(contentType, "text", "html", {"charset": "utf-8"});
   Expect.equals("text/html; charset=utf-8", contentType.toString());
+  Expect.throws(() => contentType.parameters["xxx"] = "yyy",
+                (e) => e is UnsupportedError);
 
   contentType = new ContentType("text",
                                 "html",
                                 parameters: {"CHARSET": "UTF-8", "xxx": "yyy"});
   check(contentType, "text", "html", {"charset": "utf-8", "xxx": "yyy"});
   String s = contentType.toString();
   bool expectedToString = (s == "text/html; charset=utf-8; xxx=yyy" ||
                            s == "text/html; xxx=yyy; charset=utf-8");
   Expect.isTrue(expectedToString);
+  Expect.throws(() => contentType.parameters["xxx"] = "yyy",
+                (e) => e is UnsupportedError);
 
   contentType = new ContentType("text",
                                 "html",
                                 charset: "ISO-8859-1",
                                 parameters: {"CHARSET": "UTF-8", "xxx": "yyy"});
   check(contentType, "text", "html", {"charset": "iso-8859-1", "xxx": "yyy"});
   s = contentType.toString();
   expectedToString = (s == "text/html; charset=iso-8859-1; xxx=yyy" ||
                       s == "text/html; xxx=yyy; charset=iso-8859-1");
   Expect.isTrue(expectedToString);
@@ skipping 145 matching lines @@
   testIfModifiedSince();
   testHost();
   testEnumeration();
   testHeaderValue();
   testContentType();
   testContentTypeCache();
   testCookie();
   testInvalidCookie();
   testHeaderLists();
 }