Add helpful comments to the animation samples (#136)

animations/lib/src/basics/03_animation_controller.dart

@@ -14,6 +14,11 @@ class AnimationControllerDemo extends StatefulWidget {
 
 class _AnimationControllerDemoState extends State<AnimationControllerDemo>
     with SingleTickerProviderStateMixin {
+  // Using the SingleTickerProviderStateMixin can ensure that our
+  // AnimationController only animates while the Widget is visible on the
+  // screen. This is a useful optimization that saves resources when the
+  // Widget is not visible.
+
   static const Duration _duration = Duration(seconds: 1);
   AnimationController controller;
 
@@ -22,20 +27,31 @@ class _AnimationControllerDemoState extends State<AnimationControllerDemo>
     super.initState();
 
     controller = AnimationController(vsync: this, duration: _duration)
+      // The Widget's build needs to be called every time the animation's
+      // value changes. So add an listener here that will call setState()
+      // and trigger the build() method to be called by the framework.
+      // If your Widget's build is relatively simple, this is a good option.
+      // However, if your build method returns a tree of child Widgets and
+      // most of them are not animated you should consider using
+      // AnimatedBuilder instead.
       ..addListener(() {
-        // Force build() to be called again
         setState(() {});
       });
   }
 
   @override
   void dispose() {
+    // AnimationController is a stateful resource that needs to be disposed when
+    // this State gets disposed.
     controller.dispose();
     super.dispose();
   }
 
   @override
   Widget build(BuildContext context) {
+    // When building the widget you can read the AnimationController's value property
+    // when building child widgets. You can also check the status to see if the animation
+    // has completed.
     return Scaffold(
       appBar: AppBar(),
       body: Center(
@@ -46,7 +62,8 @@ class _AnimationControllerDemoState extends State<AnimationControllerDemo>
               constraints: BoxConstraints(maxWidth: 200),
               child: Text(
                 '${controller.value.toStringAsFixed(2)}',
-                style: Theme.of(context).textTheme.display3,
+                style: Theme.of(context).textTheme.display2,
+                textScaleFactor: 1 + controller.value,
               ),
             ),
             RaisedButton(