Fix HttpHeaders.date and add HttpHeaders.ifModifiedSince.

runtime/bin/http_impl.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.
 
 class _HttpHeaders implements HttpHeaders {
   _HttpHeaders() : _headers = new Map<String, List<String>>();
 
   List<String> operator[](String name) {
     name = name.toLowerCase();
     return _headers[name];
@@ skipping 63 matching lines @@
   }
 
   int get port() => _port;
 
   void set port(int port) {
     _checkMutable();
     _port = port;
     _updateHostHeader();
   }
 
+  Date get ifModifiedSince() {
+    List<String> values = _headers["if-modified-since"];
+    if (values != null) {
+      try {
+        return _HttpUtils.parseDate(values[0]);
+      } catch (Exception e) {
+        return null;
+      }
+    }
+    return null;
+  }
+
+  void set ifModifiedSince(Date ifModifiedSince) {
+    _checkMutable();
+    // Format "ifModifiedSince" header with date in Greenwich Mean Time (GMT).
+    String formatted = _HttpUtils.formatDate(ifModifiedSince.toUtc());
+    _set("if-modified-since", formatted);
+  }
+
   Date get date() {
     List<String> values = _headers["date"];
     if (values != null) {
       try {
         return _HttpUtils.parseDate(values[0]);
       } catch (Exception e) {
         return null;
       }
     }
     return null;
   }
 
   void set date(Date date) {
     _checkMutable();
     // Format "Date" header with date in Greenwich Mean Time (GMT).
-    String formatted = _HttpUtils.formatDate(expires.toUtc());
+    String formatted = _HttpUtils.formatDate(date.toUtc());
     _set("date", formatted);
   }
 
   Date get expires() {
     List<String> values = _headers["expires"];
     if (values != null) {
       try {
         return _HttpUtils.parseDate(values[0]);
       } catch (Exception e) {
         return null;
@@ skipping 34 matching lines @@
         throw new HttpException("Unexpected type for header named $name");
       }
     } else if (name.toLowerCase() == "expires") {
       if (value is Date) {
         expires = value;
       } else if (value is String) {
         _set("expires", value);
       } else {
         throw new HttpException("Unexpected type for header named $name");
       }
+    } else if (name.toLowerCase() == "if-modified-since") {
+      if (value is Date) {
+        ifModifiedSince = value;
+      } else if (value is String) {
+        _set("if-modified-since", value);
+      } else {
+        throw new HttpException("Unexpected type for header named $name");
+      }
     } else if (name.toLowerCase() == "host") {
       int pos = value.indexOf(":");
       if (pos == -1) {
         _host = value;
         _port = HttpClient.DEFAULT_HTTP_PORT;
       } else {
         if (pos > 0) {
           _host = value.substring(0, pos);
         } else {
           _host = null;
@@ skipping 11 matching lines @@
       }
     } else if (name.toLowerCase() == "content-type") {
       _set("content-type", value);
     } else {
       name = name.toLowerCase();
       List<String> values = _headers[name];
       if (values == null) {
         values = new List<String>();
         _headers[name] = values;
       }
-      values.add(value.toString());
+      if (value is Date) {
+        values.add(_HttpUtils.formatDate(value));
+      } else {
+        values.add(value.toString());
+      }
     }
   }
 
   void _set(String name, String value) {
     name = name.toLowerCase();
     List<String> values = new List<String>();
     _headers[name] = values;
     values.add(value);
   }
 
@@ skipping 1939 matching lines @@
 
 
 class _RedirectInfo implements RedirectInfo {
   const _RedirectInfo(int this.statusCode,
                       String this.method,
                       Uri this.location);
   final int statusCode;
   final String method;
   final Uri location;
 }