Make getRootElement() public

Fixes emberjs#284

addon-test-support/@ember/test-helpers/dom/get-root-element.js

@@ -1,7 +1,9 @@
 import { getContext } from '../setup-context';
 
 /**
-  @private
+  Get the root element of the application under test (usually `#ember-testing`)
+
+  @public
   @returns {Element} the root element
 */
 export default function getRootElement() {

addon-test-support/@ember/test-helpers/index.js

@@ -31,5 +31,6 @@ export { default as triggerEvent } from './dom/trigger-event';
 export { default as triggerKeyEvent } from './dom/trigger-key-event';
 export { default as fillIn } from './dom/fill-in';
 export { default as waitFor } from './dom/wait-for';
+export { default as getRootElement } from './dom/get-root-element';
 export { default as find } from './dom/find';
 export { default as findAll } from './dom/find-all';

tests/unit/dom/get-root-element-test.js

@@ -0,0 +1,36 @@
+import { module, test } from 'qunit';
+import { getRootElement, setupContext, teardownContext } from '@ember/test-helpers';
+import hasEmberVersion from 'ember-test-helpers/has-ember-version';
+
+module('DOM Helper: getRootElement', function(hooks) {
+  if (!hasEmberVersion(2, 4)) {
+    return;
+  }
+
+  let context;
+
+  hooks.beforeEach(function() {
+    context = {};
+  });
+
+  hooks.afterEach(async function() {
+    if (context.owner) {
+      await teardownContext(context);
+    }
+
+    document.getElementById('ember-testing').innerHTML = '';
+  });
+
+  test('works with context set', async function(assert) {
+    await setupContext(context);
+
+    let fixture = document.querySelector('#ember-testing');
+    assert.equal(getRootElement(), fixture);
+  });
+
+  test('throws without context set', function(assert) {
+    assert.throws(() => {
+      getRootElement();
+    }, /Must setup rendering context before attempting to interact with elements/);
+  });
+});