babel/packages/babel-plugin-transform-strict-mode/src/index.js

import * as t from "babel-types";

export default function () {
  return {
    visitor: {
      Program(path) {
        let { node } = path;
        
        for (let directive of (node.directives: Array<Object>)) {
          if (directive.value.value === "use strict") return;
        }

        path.unshiftContainer("directives", t.directive(t.directiveLiteral("use strict")));
      }
    }
  };
}