ecmascript-2017 - ES6 await 是保留字

Question

我有这样的示例代码：

import net from 'net'
import B from 'bluebird'

class Test {
    constructor() {}
    async start() {
        return await new B((resolve, reject) => {
            try {
                this.socketClient = net.connect(4724);
                // Windows: the socket errors out when ADB restarts. Let's catch it to avoid crashing.
                this.socketClient.on('error', (err) => {
                    if (!this.ignoreUnexpectedShutdown) {
                        throw new Error(`Android bootstrap socket crashed: ${err}`);
                    }
                });
                this.socketClient.once('connect', () => {
                    log.info("Android testbundle socket is now connected");
                    resolve();
                });
            } catch (err) {
                reject(err);
            }
        })
    }
}
let t = new Test()
await t.start()

但是当我转换成 es5 代码时，我得到了这个错误：

SyntaxError: src/test.js: await is a reserved word (34:0)
  32 |
  33 | let t = new Test()
> 34 | await t.start()

我知道await应该对应async，但是start()方法被标记为async，还是报错。如何解决？