[BUGFIX beta] Observers after Dependent Key Change

Currently when a property is overriden in an extended class and its
dependent keys change, observers watching that property will still fire
for changes made to the old set of dependent keys.

```js
var count = 0;

var MyClass = Ember.Object.extend({
    prop1: 'foo',
    content: Ember.computed.alias('prop1'),
    incrementCount: Ember.observer('content', function () { count++; })
});

var obj = MyClass.extend({
    prop2: Ember.computed.alias('prop1'),
    content: Ember.computed.alias('prop2') // redefine content with diff depKey
}).create();

Ember.run(function() {
    obj.set('prop1', 'bar');
});

// assert(count === 1); // Fails, should be 1 but in fact is 2
```

packages/ember-metal/lib/property_events.js

@@ -178,6 +178,9 @@ function iterDeps(method, obj, deps, depKey, seen, meta) {
     keys = keysOf(deps);
     for (i = 0; i < keys.length; i++) {
       key = keys[i];
+
+      if (!deps[key]) { continue; }
+
       possibleDesc = obj[key];
       desc = (possibleDesc !== null && typeof possibleDesc === 'object' && possibleDesc.isDescriptor) ? possibleDesc : undefined;
 

packages/ember-metal/tests/alias_test.js

@@ -42,6 +42,42 @@ QUnit.test('basic lifecycle', function() {
   equal(m.readableDeps('foo.faz').bar, 0);
 });
 
+QUnit.test('old dependent keys should not trigger property changes', function() {
+  var obj1 = Object.create(null);
+  defineProperty(obj1, 'foo', null, null);
+  defineProperty(obj1, 'bar', alias('foo'));
+  defineProperty(obj1, 'baz', alias('foo'));
+  defineProperty(obj1, 'baz', alias('bar')); // redefine baz
+  addObserver(obj1, 'baz', incrementCount);
+
+  set(obj1, 'foo', 'FOO');
+  equal(count, 1);
+
+  removeObserver(obj1, 'baz', incrementCount);
+
+  set(obj1, 'foo', 'OOF');
+  equal(count, 1);
+});
+
+QUnit.test('overridden dependent keys should not trigger property changes', function() {
+  var obj1 = Object.create(null);
+  defineProperty(obj1, 'foo', null, null);
+  defineProperty(obj1, 'bar', alias('foo'));
+  defineProperty(obj1, 'baz', alias('foo'));
+  addObserver(obj1, 'baz', incrementCount);
+
+  var obj2 = Object.create(obj1);
+  defineProperty(obj2, 'baz', alias('bar')); // override baz
+
+  set(obj2, 'foo', 'FOO');
+  equal(count, 1);
+
+  removeObserver(obj2, 'baz', incrementCount);
+
+  set(obj2, 'foo', 'OOF');
+  equal(count, 1);
+});
+
 QUnit.test('begins watching alt key as soon as alias is watched', function() {
   defineProperty(obj, 'bar', alias('foo.faz'));
   addObserver(obj, 'bar', incrementCount);