chore: rename modules/examples to modules/playground

The directory contains code authored in a style that makes it transpilable to dart. As such, these are not idiomatic examples of Angular 2 usage.

The main purpose of this directory is to enable experimentation with Angular within the angular/angular repository.

Closes #4342

Closes #4639

modules/playground/e2e_test/hello_world/hello_world_spec.dart

@@ -0,0 +1,3 @@
+library playground.e2e_test.hello_world.hello_world_spec;
+
+main() {}

modules/playground/e2e_test/hello_world/hello_world_spec.ts

@@ -0,0 +1,34 @@
+import {verifyNoBrowserErrors} from 'angular2/src/testing/e2e_util';
+
+describe('hello world', function() {
+
+  afterEach(verifyNoBrowserErrors);
+
+  describe('hello world app', function() {
+    var URL = 'playground/src/hello_world/index.html';
+
+    it('should greet', function() {
+      browser.get(URL);
+
+      expect(getComponentText('hello-app', '.greeting')).toEqual('hello world!');
+    });
+
+    it('should change greeting', function() {
+      browser.get(URL);
+
+      clickComponentButton('hello-app', '.changeButton');
+      expect(getComponentText('hello-app', '.greeting')).toEqual('howdy world!');
+    });
+  });
+
+});
+
+function getComponentText(selector, innerSelector) {
+  return browser.executeScript('return document.querySelector("' + selector + '").querySelector("' +
+                               innerSelector + '").textContent');
+}
+
+function clickComponentButton(selector, innerSelector) {
+  return browser.executeScript('return document.querySelector("' + selector + '").querySelector("' +
+                               innerSelector + '").click()');
+}