`setWatch` interrupt loop for MPU6050 seems to block async processes on Espruino.

Continuing from https://github.com/espruino/EspruinoDocs/issues/367#issuecomment-306120068,

I tried the "falling" edge suggestion, but the behavior seems the same, the HTTP server never responds, and I never see the ------ dmpYawPitchRoll output in Espruino console.

Here's the code I'm trying:

function getMPU() {
    I2C1.setup({scl:B6,sda:B7, bitrate: 100000});

    var MPU6050 = require("MPU6050");
    console.log(MPU6050);

    var mpu = MPU6050.connect(I2C1);

    return mpu
}

function connectToWifi(wifiName, options) {
    var resolve, reject
    var promise = new Promise(function(res, rej) {resolve = res; reject = rej})

    var wifi = require("EspruinoWiFi");

    wifi.connect(wifiName, options, function(err) {
        if (err) reject(err);
        resolve();
    });

    return promise
}

connectToWifi("wifi", { password : "password" })
.then(function() {
    const mpu = getMPU();
    const dmp = require('MPU6050_DMP').create(mpu, 3);
    const http = require("http");

    let dmpYawPitchRoll = null

    // This loop locks up the espruino: https://github.com/espruino/EspruinoDocs/issues/367
    function dmpLoop() {
        const dmpData = dmp.getData()
        if (dmpData !== undefined) {
            //console.log(
                dmpYawPitchRoll = dmp.getYawPitchRoll(dmpData)
            //)
        }
    }
    setWatch(dmpLoop, B5, { repeat:true, edge:'falling' });

    http.createServer(function (req, res) {
        res.writeHead(200);
        let result = {
            acceleration: mpu.getAcceleration(), // returns an [x,y,z] array with raw accl. data
            gravity: mpu.getGravity(),  // returns acceleration array in G's
            rotation: mpu.getRotation(), // returns an [x,y,z] array with raw gyro data
            degreesPerSecond: mpu.getDegreesPerSecond(), // returns gyro array in degrees/s
        }
        result = JSON.stringify(result)
        res.end(result);

        // This never fires, HTTP response is never sent to computer
        console.log(' ------ dmpYawPitchRoll:', dmpYawPitchRoll)

    }).listen(80);

    const wifi = require("EspruinoWiFi");
    wifi.getIP(function(err, info) {
        if (err) throw err

        console.log('IP:', info.ip)
    });
});

function onInit() {
    console.log('Espruino started!');
}

Making an HTTP request from the computer to Espruino doesn't work. If you uncomment the console.log calls, the setWatch loop is working, and I will see that output repeatedly in console.