chore(tests): Enable dev mode in all Angular tests.

Closes #5106

modules/angular2/test/dev_mode_spec.ts

@@ -0,0 +1,20 @@
+import {
+  AsyncTestCompleter,
+  beforeEach,
+  ddescribe,
+  describe,
+  expect,
+  iit,
+  inject,
+  it,
+  xdescribe,
+  xit
+} from 'angular2/testing_internal';
+
+import {assertionsEnabled} from 'angular2/src/core/facade/lang';
+
+export function main() {
+  describe('dev mode', () => {
+    it('is enabled in our tests by default', () => { expect(assertionsEnabled()).toBe(true); });
+  });
+}