CodeMirror keymap isn't working inside brackets-app

[njx]

1. Open brackets-app
2. In the empty editor, type some text
3. Hit Cmd-A to select all

Result: Doesn't work. Some other keyboard shortcuts, like Cmd-Z, appear to work, but they're actually going through the browser, not CodeMirror, and so don't actually do the right thing. This blocks implementation of things like figuring out whether files are dirty and need to be saved.