introduce Ember._Cache still private API

packages/ember-metal/lib/cache.js

@@ -0,0 +1,55 @@
+import dictionary from 'ember-metal/dictionary';
+export default Cache;
+
+function Cache(limit, func) {
+  this.store  = dictionary(null);
+  this.size   = 0;
+  this.misses = 0;
+  this.hits   = 0;
+  this.limit  = limit;
+  this.func   = func;
+}
+
+var FALSE     = function() { };
+var ZERO      = function() { };
+var UNDEFINED = function() { };
+var NULL      = function() { };
+
+Cache.prototype = {
+  set: function(key, value) {
+    if (this.limit > this.size) {
+      this.size ++;
+      if (value === undefined) {
+        this.store[key] = UNDEFINED;
+      } else {
+        this.store[key] = value;
+      }
+    }
+
+    return value;
+  },
+
+  get: function(key) {
+    var value = this.store[key];
+
+    if (value === undefined) {
+      this.misses ++;
+      value = this.set(key, this.func(key));
+    } else if (value === UNDEFINED) {
+      this.hits ++;
+      value = UNDEFINED;
+    } else {
+      this.hits ++;
+      // nothing to translate
+    }
+
+    return value;
+  },
+
+  purge: function() {
+    this.store  = dictionary(null);
+    this.size   = 0;
+    this.hits   = 0;
+    this.misses = 0;
+  }
+};