[Containment] Disallow composition in immutable objects

Disallow composition in objects which cannot be created, under
the rationale that creatable objects must also be immutable.
WTD-1098.

platform/containment/src/CompositionMutabilityPolicy.js

@@ -0,0 +1,30 @@
+/*global define*/
+
+define(
+    [],
+    function () {
+        "use strict";
+
+        /**
+         * Disallow composition changes to objects which are not mutable.
+         * @constructor
+         */
+        function CompositionMutabilityPolicy() {
+            return {
+                /**
+                 * Is the type identified by the candidate allowed to
+                 * contain the type described by the context?
+                 * @param {Type} candidate the type of domain object
+                 */
+                allow: function (candidate) {
+                    // Equate creatability with mutability; that is, users
+                    // can only modify objects of types they can create, and
+                    // vice versa.
+                    return candidate.hasFeature('creation');
+                }
+            };
+        }
+
+        return CompositionMutabilityPolicy;
+    }
+);