babel/packages/babel-plugin-transform-object-assign/src/index.js

import { declare } from "@babel/helper-plugin-utils";

export default declare(api => {
  api.assertVersion(7);

  return {
    visitor: {
      CallExpression: function(path, file) {
        if (path.get("callee").matchesPattern("Object.assign")) {
          path.node.callee = file.addHelper("extends");
        }
      },
    },
  };
});