incorrect return type of async function ruined tsc 1.6.2

/usr/local/lib/node_modules/typescript/lib/tsc.js:29825

```
async function abc():number{
    return 123;
}
```

```
/usr/local/lib/node_modules/typescript/lib/tsc.js:29825
                throw e;
                      ^
TypeError: Cannot read property 'flags' of undefined
    at isRelatedTo (/usr/local/lib/node_modules/typescript/lib/tsc.js:13951:56)
```
