This is a chrome extension to quickly insert mock credit cards into braintree credit card fields.
In chrome, go to chrome://extensions/
Click Load unpacked extension...
Select the directory that you downloaded this codebase to.
In pages with an iframe#braintree-hosted-field-number element, a green page icon will appear in the url bar. You can click that to insert a mock credit card.
This chrome extension has access to insert javascript into any pages or iframes with the url "https://assets.braintreegateway.com/*". You should be wary, and read the source code.