[BUGFIX release] Fixes #15154

Usage of `Array#includes` is not supported by IE11.

(cherry picked from commit 6d3b250)

packages/ember-metal/lib/error_handler.js

@@ -6,7 +6,7 @@ let getStack = error => {
   let stack = error.stack;
   let message = error.message;
 
-  if (stack && !stack.includes(message)) {
+  if (stack && stack.indexOf(message) === -1) {
     stack = `${message}\n${stack}`;
   }