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

const OBJECT_ASSIGN = 'ObjectAssign';

export default function ({types: t}) {
  return {
    visitor: {
      Program: {
        enter(path, {file}) {
          file.set(OBJECT_ASSIGN, false);
        },

        exit(path, {file, opts}) {
          if (typeof opts === 'string') {
            throw new Error(`Configuring module specifier with a string is no longer supported. Configure with { "moduleSpecifier": "${opts}" } instead of "${opts}".`);
          }

          const { moduleSpecifier = 'object-assign' } = opts;

          if (!file.get(OBJECT_ASSIGN) && !path.scope.hasBinding(moduleSpecifier)) {
            return;
          }

          const declar = t.importDeclaration([
            t.importDefaultSpecifier(file.get(OBJECT_ASSIGN)),
          ], t.stringLiteral(moduleSpecifier));

          path.node.body.unshift(declar);
        }
      },

      CallExpression(path, {file}) {
        if (path.get('callee').matchesPattern('Object.assign')) {

          if (!file.get(OBJECT_ASSIGN)) {
            file.set(OBJECT_ASSIGN, path.scope.generateUidIdentifier('objectAssign'));
          }

          path.node.callee = file.get(OBJECT_ASSIGN);
        }
      }
    }
  };
}