Fixes #965 as long as requestUpdate is called before setting other properties

[sorvell]

It is now possible to set a computed reflecting property in another property's setter as long as requestUpdate is called before setting the computed property. Previous to this change, the following construction would fail:

// snippit from element class
@property({type: String, reflect: true}) b: string;

@property()
set a(value) {
  const oldValue = this.a;
  this._a = value;
  this.b = 'b';
  this.requestUpdate('a', oldValue);
}

// user code
el.setAttribute('a', 'a');
await el.updateComplete;
console.assert(el.getAttribute('b') == 'b') // false =(

With this change, the above code will still fail but it can be trivially changed so that it works by ensuring the other property is set after calling requestUpdate.

// updated snippit from class with everything the same except 
// where requestUpdate is called in the setter
set a(value) {
  const oldValue = this.a;
  this._a = value; 
  this.requestUpdate('a', oldValue); // ok to set a reflecting property after this
  this.b = 'b';
}

// user code
el.setAttribute('a', 'a');
await el.updateComplete;
console.assert(el.getAttribute('b') == 'b') // true =)