Fix HttpHeaders.date and add HttpHeaders.ifModifiedSince.

runtime/bin/http.dart

 // Copyright (c) 2012, 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.
 
 /**
  * HTTP status codes.
  */
 interface HttpStatus {
   static final int CONTINUE = 100;
   static final int SWITCHING_PROTOCOLS = 101;
@@ skipping 268 matching lines @@
   /**
    * Disable folding for the header named [name] when sending the HTTP
    * header. By default, multiple header values are folded into a
    * single header line by separating the values with commas. The
    * Set-Cookie header has folding disabled by default.
    */
   void noFolding(String name);
 
   /**
    * Gets and sets the date. The value of this property will
-   * reflect the "Date" header
+   * reflect the "Date" header.
    */
   Date date;
 
   /**
    * Gets and sets the expiry date. The value of this property will
-   * reflect the "Expires" header
+   * reflect the "Expires" header.
    */
   Date expires;
 
   /**
+   * Gets and sets the 'if-modified-since' date. The value of this property will
+   * reflect the "if-modified-since" header.
+   */
+  Date ifModifiedSince;
+
+  /**
    * Gets and sets the host part of the "Host" header for the
    * connection.
    */
   String host;
 
   /**
    * Gets and sets the port part of the "Host" header for the
    * connection.
    */
   int port;
@@ skipping 618 matching lines @@
 class RedirectLimitExceededException extends RedirectException {
   const RedirectLimitExceededException(List<RedirectInfo> redirects)
       : super("Redirect limit exceeded", redirects);
 }
 
 
 class RedirectLoopException extends RedirectException {
   const RedirectLoopException(List<RedirectInfo> redirects)
       : super("Redirect loop detected", redirects);
 }