Add __esModule property to exports object

Fixes: #442

build/babel-plugin.js

@@ -29,6 +29,19 @@ module.exports = ({ types: t }) => ({
                       )
                     )
                   ),
+                  t.expressionStatement(
+                    t.callExpression(
+                      t.memberExpression(
+                        t.identifier('Object'), t.identifier('defineProperty')),
+                      [
+                        t.identifier('exports'),
+                        t.stringLiteral('__esModule'),
+                        t.objectExpression([
+                          t.objectProperty(t.identifier('value'), t.booleanLiteral(true))
+                        ])
+                      ]
+                    )
+                  ),
                   t.expressionStatement(
                     t.assignmentExpression('=',
                       expr.node.left, t.identifier('exports')

build/rollup-plugin.js

@@ -7,6 +7,7 @@ export default function tweakDefault() {
         var matches = /^(exports(?:\['default']|\.default)) = (.*);$/.exec(line);
         if (matches) {
           lines[i] = 'module.exports = exports = ' + matches[2] + ';\n' +
+                     'Object.defineProperty(exports, "__esModule", { value: true });\n' +
                      matches[1] + ' = exports;';
           break;
         }