incorrect return type of async function ruined tsc 1.6.2

[cevek]

/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)

[danquirk]

At a glance, every single test we have using async returns Promise<void>. Really need to beef that up for non-void Promises as well as other cases like this one.

[mhegazy]

+1 for more tests