Static properties on components break tree-shaking #245

In our app we are finding that any time a component that uses styled-components has a static property added to it, that component is always included in the bundle even if it's not imported, i.e. tree shaking does not work. For example:

const Wrapper = styled.div`
    color: blue;
`

function MyComponent() {
    return <Wrapper>hi</Wrapper>
}
// this breaks tree-shaking
MyComponent.displayName = 'Foo'

In some cases we're able to remove the displayName or use default function argument values instead of defaultProps, but there are other cases where we do need to use these properties.

This seems to be caused by the way terser (or uglify-js) removes dead code and isn't really an issue in babel-plugin-styled-components per se, but it would be great if babel-plugin-styled-components could include an option to address this.