Compiler allows async function expressions in ES3/ES5

[yortus]

TypeScript Version:

1.8

Code

// file: index.ts
declare class Promise<T> {/***/}
async function foo() { return 42; } // ERROR: Async functions are only available in ES6+
let bar = async function () { return 42; } // OK, but should be an error
let baz = async () => 42; // OK, but should be an error

// file: tsconfig.json
{
 "compilerOptions": {
 "target": "es5"
 }
}

Expected behavior:
The bar and baz lines should also get error TS1311 'Async functions are only available in ES6 or higher'.

Actual behavior:
The bar and baz lines compile without errors.

[zpdDG4gta8XKpMCd]

#8581 might me related

[zpdDG4gta8XKpMCd]

also might relate: #8584