mark barback test "too_many_open_files_test" as flaky

tests/compiler/dart2js/source_mapping_test.dart

Index: tests/compiler/dart2js/source_mapping_test.dart
diff --git a/tests/compiler/dart2js/source_mapping_test.dart b/tests/compiler/dart2js/source_mapping_test.dart
index b219719c67ecfdf73c74a0827ec1218819df1d5d..752d05a9bf1967cf10fde3513c27cb0345e168ce 100644
--- a/tests/compiler/dart2js/source_mapping_test.dart
+++ b/tests/compiler/dart2js/source_mapping_test.dart
@@ -41,8 +41,8 @@ void testSourceMapLocations(String codeWithMarkers) {
       int expectedLocation = expectedLocations[i];
       if (!locations.contains(expectedLocation)) {
         int originalLocation = expectedLocation + i;
-        SourceFile sourceFileWithMarkers = new StringSourceFile('<test script>',
-                                                          codeWithMarkers);
+        SourceFile sourceFileWithMarkers =
+            new StringSourceFile('<test script>', codeWithMarkers);
         String message = sourceFileWithMarkers.getLocationMessage(
             'Missing location', originalLocation, originalLocation + 1, true,
             (s) => s);