If you run into any problems, please file a new bug. Rather than waiting for a fix, we encourage you to consider submitting a PR yourself. See our contributing guide for details.

Adding more widgets to lib/flutter/core_widgets.dart and lib/flutter/material_widgets.dart is welcome.

When contributing code, ensure that flutter test --coverage; lcov --list coverage/ continues to show 100% test coverage, and update test_coverage/bin/test_coverage.dart with the appropriate expectations to prevent future coverage regressions. (That program is run by


To run the tests, use flutter test.

To run the web tests, use flutter test --platform=chrome. If there is code that only runs on the web target, mark each such line by ending it with the string // dead code on VM target. This will exclude that line from the coverage calculation.

Golden tests are only run against the Flutter master channel and only run on Linux, since minor rendering differences are expected on different platforms and on different versions of Flutter.

Golden test failures

When golden tests fail, several files will be created in a test/failures directory.

The suffix of each file briefly describes its contents:

  • _masterImage: The current golden against which comparisons are being made.
  • _testImage: The image generated by this run of the test.
  • _isolatedDiff: An image where only the pixels that differ between the master and test images are rendered.
  • _maskedDiff: An image where the pixels that differ between the master and test images are rendered in black (isolatedDiff + maskedDiff = testImage).

Check the _testImage and the Diffs and see if the changes are valid. From time to time, a new dithering or anti-aliasing algorithm will land in Flutter that will invalidate a bunch of goldens, but not really break the rendering.

In this case, you may update the golden files.

Updating golden files

Once you've validated that the new goldens are correct, you may update them by calling --update-goldens.

That command updates the reference golden images in the test/goldens directory.

Read more about Golden matching in package:flutter_test API docs.