[BUGFIX] Changed backburner's error handler to use dispatchError in…

…stead of `onError`. This is so that backburner errors can be caught by the `Test.adapter`. Fixes #14864.

packages/ember-metal/lib/run_loop.js

@@ -2,7 +2,7 @@ import { GUID_KEY } from 'ember-utils';
 import { assert } from './debug';
 import { isTesting } from './testing';
 import {
-  getOnerror,
+  dispatchError,
   setOnerror
 } from './error_handler';
 import {
@@ -21,7 +21,7 @@ function onEnd(current, next) {
 
 const onErrorTarget = {
   get onerror() {
-    return getOnerror();
+    return dispatchError;
   },
   set onerror(handler) {
     return setOnerror(handler);

packages/ember-metal/tests/run_loop/onerror_test.js

@@ -1,5 +1,7 @@
 import run from '../../run_loop';
 import { setOnerror, getOnerror } from '../../error_handler';
+import { isTesting, setTesting } from '../../testing';
+import { Test, QUnitAdapter }  from 'ember-testing';
 
 QUnit.module('system/run_loop/onerror_test');
 
@@ -23,13 +25,40 @@ QUnit.test('With Ember.onerror undefined, errors in Ember.run are thrown', funct
 QUnit.test('With Ember.onerror set, errors in Ember.run are caught', function () {
   let thrown = new Error('Boom!');
   let original = getOnerror();
+  let originalAdapter = Test.adapter;
+  let originalIsTesting = isTesting();
 
   let caught;
   setOnerror(error => { caught = error; });
+  Test.adapter = null;
+  setTesting(false);
+
   try {
     run(() => { throw thrown; });
   } finally {
     setOnerror(original);
+    Test.adapter = originalAdapter;
+    setTesting(originalIsTesting);
+  }
+
+  deepEqual(caught, thrown);
+});
+
+QUnit.test('With Ember.Test.adapter set, errors in Ember.run are caught', function () {
+  let thrown = new Error('Boom!');
+  let originalAdapter = Test.adapter;
+
+  let caught;
+  Test.adapter = QUnitAdapter.create({
+    exception(error) {
+      caught = error;
+    }
+  });
+
+  try {
+    run(() => { throw thrown; });
+  } finally {
+    Test.adapter = originalAdapter;
   }
 
   deepEqual(caught, thrown);