docs(dev guide): lifecycle-hooks - updated dart/ts code and new dart prose

Mainly copyedits, but also

- Dart .jade extends TS .jade file with minor overrides
- Significant update of example code (so it matches the ts example in its
appearance and behavior).
- Tweaks to Dart code.
- A few extra/corrected mixin definitions in `_util-fns.jade`.

Author: chalin

public/_includes/_util-fns.jade

@@ -1,5 +1,17 @@
 //- Mixins and associated functions
 
+- var _docsFor = 'ts'; // or 'dart' or 'js'.
+
+mixin ifDocsFor(lang)
+  if _docsFor.toLowerCase() === lang.toLowerCase()
+    block
+
+mixin adjExPath(path)
+  if adjustExamplePath
+    | #{adjustExamplePath(path)}
+  else
+    | #{path}
+  
 mixin includeShared(filePath, region)
   - var newPath = translatePath(filePath, region);
   !=partial(newPath)

public/docs/_examples/lifecycle-hooks/dart/lib/after_content_component.dart

@@ -0,0 +1,115 @@
+// #docplaster
+// #docregion
+import 'package:angular2/core.dart';
+
+import 'logger_service.dart';
+
+//////////////////
+@Component(
+    selector: 'my-child',
+    template: '<input [(ngModel)]="hero">')
+class ChildComponent {
+  String hero = 'Magneta';
+}
+
+//////////////////////
+@Component(
+    selector: 'after-content',
+// #docregion template
+    template: '''
+      <div>-- projected content begins --</div>
+        <ng-content></ng-content>
+      <div>-- projected content ends --</div>
+      <p *ngIf="comment != null" class="comment">{{comment}}</p>
+    '''
+// #enddocregion template
+    )
+// #docregion hooks
+class AfterContentComponent implements AfterContentChecked, AfterContentInit {
+  String _prevHero = '';
+  String comment = '';
+
+  // Query for a CONTENT child of type `ChildComponent`
+  @ContentChild(ChildComponent) ChildComponent contentChild;
+
+// #enddocregion hooks
+  final LoggerService _logger;
+
+  AfterContentComponent(this._logger) {
+    _logIt('AfterContent constructor');
+  }
+
+// #docregion hooks
+  ngAfterContentInit() {
+    // contentChild is set after the content has been initialized
+    _logIt('AfterContentInit');
+    _doSomething();
+  }
+
+  ngAfterContentChecked() {
+    // contentChild is updated after the content has been checked
+    if (_prevHero == contentChild?.hero) {
+      _logIt('AfterContentChecked (no change)');
+    } else {
+      _prevHero = contentChild?.hero;
+      _logIt('AfterContentChecked');
+      _doSomething();
+    }
+  }
+// #enddocregion hooks
+// #docregion do-something
+  /// This surrogate for real business logic; sets the `comment`
+  void _doSomething() {
+    comment = contentChild.hero.length > 10 ? "That's a long name" : '';
+  }
+// #enddocregion do-something
+
+  void _logIt(String method) {
+    var child = contentChild;
+    var message = "${method}: ${child?.hero ?? 'no'} child content";
+    _logger.log(message);
+  }
+// #docregion hooks
+  // ...
+}
+// #enddocregion hooks
+
+//////////////
+@Component(
+    selector: 'after-content-parent',
+// #docregion parent-template
+    template: '''
+      <div class="parent">
+        <h2>AfterContent</h2>
+
+        <div *ngIf="show">
+          <after-content>
+            <my-child></my-child>
+          </after-content>
+        </div>
+
+        <h4>-- AfterContent Logs --</h4>
+        <p><button (click)="reset()">Reset</button></p>
+        <div *ngFor="let msg of logs">{{msg}}</div>
+      </div>
+    ''',
+// #enddocregion parent-template
+    styles: const ['.parent {background: burlywood}'],
+    providers: const [LoggerService],
+    directives: const [AfterContentComponent, ChildComponent])
+class AfterContentParentComponent {
+  final LoggerService _logger;
+  bool show = true;
+
+  AfterContentParentComponent(this._logger);
+
+  List<String> get logs => _logger.logs;
+
+  void reset() {
+    logs.clear();
+    // quickly remove and reload AfterViewComponent which recreates it
+    show = false;
+    _logger.tick().then((_) { show = true; });
+  }
+
+}