fix failing test

Author: evaneus

test/unit/code-gen/index-test.js

@@ -8,7 +8,7 @@ describe('declaration', () => {
     describe('js', () => {
       it('returns a custom element declaration', () => {
         let d = new mod.ElementDeclaration('my-tag');
-        expect(d.js).to.eql(`import MyTag from 'my-tag';\ncustomElements.define('my-tag', MyTag);`);
+        expect(d.js).to.eql(`import MyTag from 'my-tag';\nif(!customElements.get('my-tag')){\ncustomElements.define('my-tag', MyTag);\n}`);
       });
     });
   });