Ensure concatenated properties are frozen.

Mutating objects on the prototype (which includes concatenated
properties) is not good. This changes the functionality of concatenated
properties to freeze the resulting array while in debug mode, to make it
clear that mutating them without slicing first is not good.

packages/ember-metal/lib/mixin.js

@@ -143,20 +143,32 @@ function giveMethodSuper(obj, key, method, values, descs) {
 
 function applyConcatenatedProperties(obj, key, value, values) {
   let baseValue = values[key] || obj[key];
+  let ret;
 
   if (baseValue) {
     if ('function' === typeof baseValue.concat) {
       if (value === null || value === undefined) {
-        return baseValue;
+        ret = baseValue;
       } else {
-        return baseValue.concat(value);
+        ret = baseValue.concat(value);
       }
     } else {
-      return makeArray(baseValue).concat(value);
+      ret = makeArray(baseValue).concat(value);
     }
   } else {
-    return makeArray(value);
+    ret = makeArray(value);
   }
+
+  runInDebug(() => {
+    // it is possible to use concatenatedProperties with strings (which cannot be frozen)
+    // only freeze objects...
+    if (typeof ret === 'object' && ret !== null) {
+      // prevent mutating `concatenatedProperties` array after it is applied
+      Object.freeze(ret);
+    }
+  });
+
+  return ret;
 }
 
 function applyMergedProperties(obj, key, value, values) {