Compiler allows async function expressions in ES3/ES5 #7334

// 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"
    }
}