docs(guide/user-input): follow-up to #1654 (#1770)

Mainly Dart-side review, following #1654:
- Updates to follow style guide
- Suites passed:
  public/docs/_examples/user-input/dart
  public/docs/_examples/user-input/ts

Note: the click-me2 component is not mentioned in the prose, maybe it
should be.

Author: chalin

public/docs/_examples/user-input/dart/lib/app_component.dart

@@ -2,7 +2,7 @@
 import 'package:angular2/core.dart';
 
 import 'click_me_component.dart';
-import 'click_me_component_2.dart';
+import 'click_me2_component.dart';
 import 'keyup_components.dart';
 import 'little_tour_component.dart';
 import 'loop_back_component.dart';
@@ -12,7 +12,7 @@ import 'loop_back_component.dart';
     templateUrl: 'app_component.html',
     directives: const [
       ClickMeComponent,
-      ClickMeComponent2,
+      ClickMe2Component,
       KeyUpComponentV1,
       KeyUpComponentV2,
       KeyUpComponentV3,

public/docs/_examples/user-input/dart/lib/click_me2_component.dart

@@ -0,0 +1,18 @@
+// #docregion
+import 'package:angular2/core.dart';
+
+@Component(
+    selector: 'click-me2',
+    template: '''
+      <button (click)="onClickMe2(\$event)">No! .. Click me!</button>
+      {{clickMessage}}''')
+class ClickMe2Component {
+  String clickMessage = '';
+  int _clicks = 1;
+
+  void onClickMe2(dynamic event) {
+    var evtMsg =
+        event != null ? ' Event target is ' + event.target.tagName : '';
+    clickMessage = ('Click #${_clicks++}. ${evtMsg}');
+  }
+}

public/docs/_examples/user-input/dart/lib/click_me_component.dart

@@ -1,19 +1,22 @@
+/* FOR DOCS ... MUST MATCH ClickMeComponent template
+// #docregion click-me-button
+  <button (click)="onClickMe()">Click me!</button>
+// #enddocregion click-me-button
+*/
+
 // #docregion
 import 'package:angular2/core.dart';
 
 // #docregion click-me-component
 @Component(
     selector: 'click-me',
     template: '''
-    // #docregion click-me-button
-    <button (click)="onClickMe()">Click me!</button>
-    // #enddocregion click-me-button
-    {{clickMessage}}''')
+      <button (click)="onClickMe()">Click me!</button>
+      {{clickMessage}}''')
 class ClickMeComponent {
   String clickMessage = '';
 
-  onClickMe() {
+  void onClickMe() {
     clickMessage = 'You are my hero!';
   }
 }
-// #enddocregion click-me-component