[BUGFIX release] [PERF] Cleanup Proxy

* remove `on(‘init`, just use the existing `init`
* remove double meta lookups
* remove observer (use existing proxy trap, and detect `key === ‘content’`
* misc cleanup

packages/ember-runtime/lib/mixins/-proxy.js

@@ -84,22 +84,13 @@ export default Mixin.create({
 
   init() {
     this._super(...arguments);
-    meta(this).setProxy();
+    let m = meta(this);
+    m.setProxy();
+    m._tag = new ProxyTag(this);
   },
 
-  _initializeTag: on('init', function() {
-    meta(this)._tag = new ProxyTag(this);
-  }),
-
-  _contentDidChange: observer('content', function() {
-    assert('Can\'t set Proxy\'s content to itself', get(this, 'content') !== this);
-    tagFor(this).contentDidChange();
-  }),
-
   isTruthy: bool('content'),
 
-  _debugContainerKey: null,
-
   willWatchProperty(key) {
     let contentKey = `content.${key}`;
     _addBeforeObserver(this, contentKey, null, contentPropertyWillChange);
@@ -126,6 +117,9 @@ export default Mixin.create({
 
   setUnknownProperty(key, value) {
     let m = meta(this);
+
+    assert('Can\'t set Proxy\'s content to itself', value !== this);
+
     if (m.proto === this) {
       // if marked as prototype then just defineProperty
       // rather than delegate
@@ -141,6 +135,11 @@ export default Mixin.create({
       !this.isController,
       { id: 'ember-runtime.controller-proxy', until: '3.0.0' }
     );
-    return set(content, key, value);
+
+    let ret = set(content, key, value);
+    if (key === 'content') {
+      tagFor(this).contentDidChange();
+    }
+    return ret;
   }
 });