test.py: Add 'Cache-Control' header to http responses: enable browsers to cache resources

tools/testing/dart/http_server.dart

Index: tools/testing/dart/http_server.dart
diff --git a/tools/testing/dart/http_server.dart b/tools/testing/dart/http_server.dart
index b222d87249bc4083b8eec01aeb3f9e47c0c0b742..720350e8d1dde1aa61df8afb19d324bf7092db38 100644
--- a/tools/testing/dart/http_server.dart
+++ b/tools/testing/dart/http_server.dart
@@ -88,6 +88,7 @@ main() {
  * test framework, such as dealing with package-root.
  */
 class TestingServers {
+  static final _CACHE_EXPIRATION_IN_SECONDS = 30;
   static final _HARMLESS_REQUEST_PATH_ENDINGS = [
     "/apple-touch-icon.png",
     "/apple-touch-icon-precomposed.png",
@@ -166,6 +167,9 @@ class TestingServers {
   void _handleFileOrDirectoryRequest(HttpRequest request,
                                      HttpResponse response,
                                      int allowedPort) {
+    // Enable browsers to cache file/directory responses.
+    response.headers.set("Cache-Control",
+                         "max-age=$_CACHE_EXPIRATION_IN_SECONDS");
     var path = _getFilePathFromRequestPath(request.uri.path);
     if (path != null) {
       var file = new File.fromPath(path);