[BUGFIX lts] Allow Ember.Test.adapter to opt out of exception handl…

…ing.

In the majority of testing frameworks (e.g. Mocha, QUnit, Jasmine) a
global error handler is attached to `window.onerror`. When an uncaught
exception is thrown, this global handler is invoked and the test suite
will properly fail.

Requiring that the provided test adapter provide an exception method
is strictly worse in nearly all cases than allowing the error to be
bubbled upwards and caught/handled by either `Ember.onerror` (if
present) or `window.onerror` (generally from the test framework).

This commit makes `Ember.Test.adapter.exception` optional. When not
present, errors during testing will be handled by existing "normal"
means.

packages/ember-testing/lib/test/adapter.js

@@ -8,7 +8,7 @@ export function getAdapter() {
 
 export function setAdapter(value) {
   adapter = value;
-  if (value) {
+  if (value && typeof value.exception === 'function') {
     setDispatchOverride(adapterDispatch);
   } else {
     setDispatchOverride(null);

packages/ember/tests/error_handler_test.js

@@ -397,6 +397,57 @@ function generateRSVPErrorHandlingTests(message, generatePromise, timeout = 10)
   });
 
   if (DEBUG) {
+    test(`${message} when Ember.Test.adapter without \`exception\` method is present - rsvp`, function(assert) {
+      assert.expect(1);
+
+      let thrown = new Error('the error');
+      Ember.Test.adapter = Ember.Test.QUnitAdapter.create({
+        exception: undefined
+      });
+
+      window.onerror = function(message) {
+        assert.pushResult({
+          result: /the error/.test(message),
+          actual: message,
+          expected: 'to include `the error`',
+          message: 'error should bubble out to window.onerror, and therefore fail tests (due to QUnit implementing window.onerror)'
+        });
+
+        // prevent "bubbling" and therefore failing the test
+        return true;
+      };
+
+      generatePromise(thrown);
+
+      // RSVP.Promise's are configured to settle within the run loop, this
+      // ensures that run loop has completed
+      return new Ember.RSVP.Promise((resolve) => setTimeout(resolve, timeout));
+    });
+
+    test(`${message} when both Ember.onerror and Ember.Test.adapter without \`exception\` method are present - rsvp`, function(assert) {
+      assert.expect(1);
+
+      let thrown = new Error('the error');
+      Ember.Test.adapter = Ember.Test.QUnitAdapter.create({
+        exception: undefined
+      });
+
+      Ember.onerror = function(error) {
+        assert.pushResult({
+          result: /the error/.test(error.message),
+          actual: error.message,
+          expected: 'to include `the error`',
+          message: 'error should bubble out to window.onerror, and therefore fail tests (due to QUnit implementing window.onerror)'
+        });
+      };
+
+      generatePromise(thrown);
+
+      // RSVP.Promise's are configured to settle within the run loop, this
+      // ensures that run loop has completed
+      return new Ember.RSVP.Promise((resolve) => setTimeout(resolve, timeout));
+    });
+
     test(`${message} when Ember.Test.adapter is present - rsvp`, function(assert) {
       assert.expect(1);