Invalid ObjectLiteral shorthand by QuotedString

```js
// test.js
var a = { '!@#$%' };
```

```
java -jar compiler.jar --language_in ECMASCRIPT6 --language_out
ECMASCRIPT3 ../test.js
```

```js
// invalid syntax
var a={"!@#$%":!@#$%};
```

Only a identifier, can say shorthand for object literal.