Make the parameters map of the HTTP HeaderValue unmodifiable

sdk/lib/io/http.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;
 
 /**
  * HTTP status codes.
  */
 abstract class HttpStatus {
@@ skipping 479 matching lines @@
  *     List<String> values = request.headers[HttpHeaders.ACCEPT];
  *     values.forEach((value) {
  *       HeaderValue v = HeaderValue.parse(value);
  *       // Use v.value and v.parameters
  *     });
  *
  * An instance of [HeaderValue] is immutable.
  */
 abstract class HeaderValue {
   /**
-   * Creates a new header value object setting the value part.
+   * Creates a new header value object setting the value and parameters.
    */
   factory HeaderValue([String value = "", Map<String, String> parameters]) {
     return new _HeaderValue(value, parameters);
   }
 
   /**
    * Creates a new header value object from parsing a header value
    * string with both value and optional parameters.
    */
   static HeaderValue parse(String value,
                            {String parameterSeparator: ";"}) {
     return _HeaderValue.parse(value, parameterSeparator: parameterSeparator);
   }
 
   /**
    * Gets the header value.
    */
   String get value;
 
   /**
    * Gets the map of parameters.
+   *
+   * This map cannot be modified. invoking any operation which would
+   * modify the map will throw [UnsupportedError].
    */
   Map<String, String> get parameters;
 
   /**
    * Returns the formatted string representation in the form:
    *
    *     value; parameter1=value1; parameter2=value2
    */
   String toString();
 }
@@ skipping 900 matching lines @@
 
 class RedirectException implements HttpException {
   final String message;
   final List<RedirectInfo> redirects;
 
   const RedirectException(String this.message,
                           List<RedirectInfo> this.redirects);
 
   String toString() => "RedirectException: $message";
 }