test: add a test verifying that the tests are run in the checked mode

modules/angular2/test/dev_mode_spec.ts

@@ -11,10 +11,23 @@ import {
   xit
 } from 'angular2/testing_internal';
 
-import {assertionsEnabled} from 'angular2/src/facade/lang';
+import {assertionsEnabled, IS_DART} from 'angular2/src/facade/lang';
 
 export function main() {
   describe('dev mode', () => {
     it('is enabled in our tests by default', () => { expect(assertionsEnabled()).toBe(true); });
   });
+
+  if (IS_DART) {
+    describe('checked mode', () => {
+      it('is enabled in our tests', () => {
+        try {
+          var s: string = <any>42;
+          expect(s).toEqual(42);  // without it, dart analyzer will complain that `s` is not used.
+          throw "should not be reached";
+        } catch (e) {
+        }
+      });
+    });
+  }
 }