How to convert a plain object into an ES6 Map?

Yes, the Map constructor takes an array of key-value pairs.

Object.entries is a new Object static method available in ES2017 (19.1.2.5).

const map = new Map(Object.entries({foo: 'bar'}));

map.get('foo'); // 'bar'

It's currently implemented in Firefox 46+ and Edge 14+ and newer versions of Chrome

If you need to support older environments and transpilation is not an option for you, use a polyfill, such as the one recommended by georg:

Object.entries = typeof Object.entries === 'function' ? Object.entries : obj => Object.keys(obj).map(k => [k, obj[k]]);

Do I really have to first convert it into an array of arrays of key-value pairs?

No, an iterator of key-value pair arrays is enough. You can use the following to avoid creating the intermediate array:

function* entries(obj) {
    for (let key in obj)
        yield [key, obj[key]];
}

const map = new Map(entries({foo: 'bar'}));
map.get('foo'); // 'bar'