docs(cheatsheet): add Dart-specific syntax & headings

Closes #5756

modules/angular2/docs/cheatsheet/lifecycle hooks.md

@@ -5,8 +5,10 @@ Directive and component change detection and lifecycle hooks
 (implemented as class methods)
 
 @cheatsheetItem
-syntax:
+syntax(js ts):
 `constructor(myService: MyService, ...) { ... }`|`constructor(myService: MyService, ...)`
+syntax(dart):
+`MyAppComponent(MyService myService, ...) { ... }`|`MyAppComponent(MyService myService, ...)`
 description:
 The class constructor is called before any other lifecycle hook. Use it to inject dependencies, but avoid any serious work here.
 
@@ -64,4 +66,4 @@ Called after every check of the component's view. Applies to components only.
 syntax:
 `ngOnDestroy() { ... }`|`ngOnDestroy()`
 description:
-Called once, before the instance is destroyed.
+Called once, before the instance is destroyed.