Add handling of HTTP header "Expires"

runtime/bin/http.dart

Index: runtime/bin/http.dart
diff --git a/runtime/bin/http.dart b/runtime/bin/http.dart
index 3a707414efd2ce355c3bb16e56b301c9ef44b64e..8a5097acceaf9c74db8da4be6934dd3378a029d5 100644
--- a/runtime/bin/http.dart
+++ b/runtime/bin/http.dart
@@ -177,6 +177,12 @@ interface HttpResponse default _HttpResponse {
   String reasonPhrase;
 
   /**
+   * Gets and sets the expiry date. The value of this property will
+   * reflect the "Expires" header
+   */
+  Date expires;
+
+  /**
    * Sets a header on the response. NOTE: If the same header name is
    * set more than once only the last value will be part of the
    * response.
@@ -366,6 +372,14 @@ interface HttpClientResponse default _HttpClientResponse {
   bool get keepAlive();
 
   /**
+   * Returns the date value for the "Expires" header. Returns null if
+   * the response has no "Expires" header. Throws a HttpException if
+   * the response has an "Expires" header which is not formatted as a
+   * valid HTTP date.
+   */
+  Date get expires();
+
+  /**
    * Returns the response headers.
    */
   Map<String, String> get headers();