Async function __awaiter allows var declarations to clobber function parameters

[yeerkkiller1]

TypeScript Version: 2.6.2, 2.7.0-dev.20171203, target: es2015

Code

async function test(url) {
    console.log(url);
}
async function fn(url) {
    await test(url);
    var url;
}

fn("test");

Expected behavior:
"test" should be logged

Actual behavior:
undefined is logged

The generated code clobbers the var declaration in the outer function, by adding another function around the code:

var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, generator) {
    return new (P || (P = Promise))(function (resolve, reject) {
        function fulfilled(value) { try { step(generator.next(value)); } catch (e) { reject(e); } }
        function rejected(value) { try { step(generator["throw"](value)); } catch (e) { reject(e); } }
        function step(result) { result.done ? resolve(result.value) : new P(function (resolve) { resolve(result.value); }).then(fulfilled, rejected); }
        step((generator = generator.apply(thisArg, _arguments || [])).next());
    });
};
function test(url) {
    return __awaiter(this, void 0, void 0, function* () {
        console.log(url);
    });
}
function fn(url) {
    return __awaiter(this, void 0, void 0, function* () {
        yield test(url);
        var url;
    });
}
fn("test");

[rbuckton]

Looks like we need to hoist var declarations out of the generator function even in ES2015.