Fix HttpHeaders.date and add HttpHeaders.ifModifiedSince.

runtime/bin/http.dart

Index: runtime/bin/http.dart
diff --git a/runtime/bin/http.dart b/runtime/bin/http.dart
index 16dc5dc17db876b2c3603dce739757a754d88589..510febf24839a7b31954b1a54785a9f4a03487dc 100644
--- a/runtime/bin/http.dart
+++ b/runtime/bin/http.dart
@@ -286,17 +286,23 @@ interface HttpHeaders default _HttpHeaders {
 
   /**
    * 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.
    */