[PERF beta] @each should remain a stable node for chains.

`@each` was designed for chaining, the special cased eager behavior + it invalidating for array changes meant that the EachProxy and chains were rebuilt every array change, instead of the leaves just changing.
Conflicts:
	packages/ember-runtime/lib/system/each_proxy.js
	packages/ember-runtime/tests/mixins/array_test.js

packages/ember-metal/lib/chains.js

@@ -192,15 +192,6 @@ function ChainNode(parent, key, value) {
       addChainWatcher(this._object, this._key, this);
     }
   }
-
-  // Special-case: the EachProxy relies on immediate evaluation to
-  // establish its observers.
-  //
-  // TODO: Replace this with an efficient callback that the EachProxy
-  // can implement.
-  if (this._parent && this._parent._key === '@each') {
-    this.value();
-  }
 }
 
 function lazyGet(obj, key) {
@@ -216,7 +207,7 @@ function lazyGet(obj, key) {
   }
 
   // Use `get` if the return value is an EachProxy or an uncacheable value.
-  if (key === '@each' || isVolatile(obj[key])) {
+  if (isVolatile(obj[key])) {
     return get(obj, key);
   // Otherwise attempt to get the cached value of the computed property
   } else {
@@ -374,12 +365,6 @@ ChainNode.prototype = {
         addChainWatcher(obj, this._key, this);
       }
       this._value  = undefined;
-
-      // Special-case: the EachProxy relies on immediate evaluation to
-      // establish its observers.
-      if (this._parent && this._parent._key === '@each') {
-        this.value();
-      }
     }
 
     // then notify chains...

packages/ember-metal/lib/expand_properties.js

@@ -22,7 +22,7 @@ var SPLIT_REGEX = /\{|\}/;
   Ember.expandProperties('{foo,bar}', echo);            //=> 'foo', 'bar'
   Ember.expandProperties('foo.{bar,baz}', echo);        //=> 'foo.bar', 'foo.baz'
   Ember.expandProperties('{foo,bar}.baz', echo);        //=> 'foo.baz', 'bar.baz'
-  Ember.expandProperties('foo.{bar,baz}.@each', echo)   //=> 'foo.bar.@each', 'foo.baz.@each'
+  Ember.expandProperties('foo.{bar,baz}.[]', echo)      //=> 'foo.bar.[]', 'foo.baz.[]'
   Ember.expandProperties('{foo,bar}.{spam,eggs}', echo) //=> 'foo.spam', 'foo.eggs', 'bar.spam', 'bar.eggs'
   Ember.expandProperties('{foo}.bar.{baz}')             //=> 'foo.bar.baz'
   ```

packages/ember-metal/tests/expand_properties_test.js

@@ -17,9 +17,10 @@ QUnit.test('Properties without expansions are unaffected', function() {
 
   expandProperties('a', addProperty);
   expandProperties('a.b', addProperty);
-  expandProperties('a.b.@each', addProperty);
+  expandProperties('a.b.[]', addProperty);
+  expandProperties('[email protected]', addProperty);
 
-  deepEqual(['a', 'a.b', 'a.b.@each'].sort(), foundProperties.sort());
+  deepEqual(['a', 'a.b', 'a.b.[]', 'a.b.@each.c'].sort(), foundProperties.sort());
 });
 
 QUnit.test('A single expansion at the end expands properly', function() {

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

@@ -26,7 +26,7 @@ import {
   sendEvent,
   hasListeners
 } from 'ember-metal/events';
-import { isWatching } from 'ember-metal/watching';
+import EachProxy from 'ember-runtime/system/each_proxy';
 
 function arrayObserversHelper(obj, target, opts, operation, notify) {
   var willChange = (opts && opts.willChange) || 'arrayWillChange';
@@ -422,13 +422,13 @@ export default Mixin.create(Enumerable, {
       }
     }
 
-    // Make sure the @each proxy is set up if anyone is observing @each
-    if (isWatching(this, '@each')) {
-      get(this, '@each');
+    if (this.__each) {
+      this.__each.arrayWillChange(this, startIdx, removeAmt, addAmt);
     }
 
     sendEvent(this, '@array:before', [this, startIdx, removeAmt, addAmt]);
 
+
     if (startIdx >= 0 && removeAmt >= 0 && get(this, 'hasEnumerableObservers')) {
       removing = [];
       lim = startIdx + removeAmt;
@@ -489,6 +489,11 @@ export default Mixin.create(Enumerable, {
     }
 
     this.enumerableContentDidChange(removeAmt, adding);
+
+    if (this.__each) {
+      this.__each.arrayDidChange(this, startIdx, removeAmt, addAmt);
+    }
+
     sendEvent(this, '@array:change', [this, startIdx, removeAmt, addAmt]);
 
     var length = get(this, 'length');
@@ -508,10 +513,6 @@ export default Mixin.create(Enumerable, {
     return this;
   },
 
-  // ..........................................................
-  // ENUMERATED PROPERTIES
-  //
-
   /**
     Returns a special object that can be used to observe individual properties
     on the array. Just get an equivalent property on this object and it will
@@ -525,15 +526,11 @@ export default Mixin.create(Enumerable, {
     @public
   */
   '@each': computed(function() {
+    // TODO use Symbol or add to meta
     if (!this.__each) {
-      // ES6TODO: GRRRRR
-      var EachProxy = requireModule('ember-runtime/system/each_proxy')['EachProxy'];
-
-      this.__each = new EachProxy({
-        content: this
-      });
+      this.__each = new EachProxy(this);
     }
 
     return this.__each;
-  })
+  }).volatile()
 });