[Bug] Observers don't work if the key contains colon ':' characters

[apellerano-pw]

🐞 Describe the Bug

Observers internally use colon in the eventName and assume there is only one colon when a split(':') operation is performed in activateObserver. By registering an observer with a key name that contains colons, you can break Ember's assumption.

🔬 Minimal Reproduction

emberObj.addObserver('key:with:colons', () => {
  console.log('I will never fire');
});

emberObj.set('key:with:colons', 'foo');

😕 Actual Behavior

The callback method never fires

🤔 Expected Behavior

The callback method fires

🌍 Environment

• Ember: - 3.16.10
• Node.js/npm: - node 10.23.0, npm 6.13.7
• OS: - mac mojave 10.14.6
• Browser: - chrome 87.0.4280.141

➕ Additional Context

The issue appears to be in the activateObserver function

  function activateObserver(target, eventName, sync = false) {
    var activeObservers = getOrCreateActiveObserversFor(target, sync);

    if (activeObservers.has(eventName)) {
      activeObservers.get(eventName).count++;
    } else {
      // the following line assumes there are no colons in the user-specified portion of the eventName
      var [path] = eventName.split(':');
      var tag = (0, _reference.combine)(getChainTagsForKey(target, path));
      activeObservers.set(eventName, {
        count: 1,
        path,
        tag,
        lastRevision: (0, _reference.value)(tag),
        suspended: false
      });
    }
  }

One gross fix would be something like

// split the string on colon, drop the final value, re-join on colon
var path = eventName.split(':').slice(0, -1).join(':')

But someone with a deeper understanding of the observer code may know of something more elegant

[rwjblue]

I think we can probably avoid splitting all together and use last index of + substr.

@pzuraq / @krisselden - what do you think?