Adding breakpoint to custom theme in Material UI Next (reactjs)

Currently it is only possible to customize the values of the predefined breakpoints. Here is an example:

const theme = createMuiTheme({
  breakpoints: {
    values: {
      xs: 0,
      sm: 450,
      md: 600,
      lg: 900,
      xl: 1200
    }
  }
});

The breakpoint names cannot be configured. There is a comment in createBreakpoints.js explaining this:

// Sorted ASC by size. That's important.
// It can't be configured as it's used statically for propTypes.
export const keys = ['xs', 'sm', 'md', 'lg', 'xl'];

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UPDATE (September 2018):

This feature has been requested and is currently under discussion. See material-ui/issues/11649