Add flow `Union` tests for `no-unused-prop-types` and `prop-types` with `import`#1570

ljharb merged 2 commits intojsx-eslint:masterjsx-eslint/eslint-plugin-react:masterfrom

justinanastos:fix/prop-types-union-flowtype-1468justinanastos/eslint-plugin-react:fix/prop-types-union-flowtype-1468Copy head branch name to clipboard