Arrow functions that return object literals can look very similar to arrow functions with brace bodies.  Some syntactic ambiguity can be avoided by disallowing block-style arrow functions in favour of ES5 function expressions.

**Outcome**

The following patterns are considered problems:
```
/*eslint arrow-body-style: ["error", "never"]*/
/*eslint-env es6*/

let foo = () => {
    return 0;
};

let foo = (retv, name) => {
    retv[name] = true;
    return retv;
};
```

The following patterns are not considered problems:
```
/*eslint arrow-body-style: ["error", "never"]*/
/*eslint-env es6*/

let foo = () => 0;

let foo = () => ({ key: 0 });
```