Ensure computation on initialization.

This handles a nuanced edge case where internal, computed properties
can be incorrect if they were interrogated between host construction and
initialization.

For example:

```javascript
// This will trigger construction (assuming the element is defined).
const element = document.createElement('my-element');

// Do things after construction, before initialization.

// This will trigger initialization (again, assuming definition).
document.body.append(element);

// Do things after initialization while connected.
```

This change set should guarantee that after initialization, during
connection — things will be correct. We cannot make such guarantees when
not connected due to lack of eventing around attribute callbacks.

Closes #143.

test/test-computed-properties.js

@@ -213,6 +213,19 @@ it('does correct NaN checking', () => {
   assert(el.count === count);
 });
 
+it('resets compute validity on initialization to catch upgrade edge cases with internal, computed properties', () => {
+  const el = document.createElement('test-element');
+  el.setAttribute('a', '1');
+  el.setAttribute('b', '2');
+  assert(el.a === undefined);
+  assert(el.b === undefined);
+  assert(Number.isNaN(el.internal.c));
+  document.body.append(el);
+  assert(el.a === 1);
+  assert(el.b === 2);
+  assert(el.internal.c === 3);
+});
+
 it('cannot be written to from host', () => {
   const el = document.createElement('test-element');
   document.body.append(el);

x-element.js

@@ -646,7 +646,7 @@ export default class XElement extends HTMLElement {
 
   static #initializeHost(host) {
     const hostInfo = XElement.#hosts.get(host);
-    const { initialized, invalidProperties } = hostInfo;
+    const { computeMap, initialized, invalidProperties } = hostInfo;
     if (initialized === false) {
       XElement.#upgradeOwnProperties(host);
       // Only reflect attributes when the element is connected.
@@ -661,6 +661,9 @@ export default class XElement extends HTMLElement {
           XElement.#setPropertyValue(host, property, property.default(host, property.initial()));
         }
         invalidProperties.add(property);
+        if (property.compute) {
+          computeMap.get(property).valid = false;
+        }
       }
       hostInfo.initialized = true;
       return true;