Landing beta changes in master for the new stable release (#747)

animations/lib/src/basics/01_animated_container.dart

@@ -18,9 +18,9 @@ class AnimatedContainerDemo extends StatefulWidget {
 }
 
 class _AnimatedContainerDemoState extends State<AnimatedContainerDemo> {
-  Color color;
-  double borderRadius;
-  double margin;
+  late Color color;
+  late double borderRadius;
+  late double margin;
 
   @override
   void initState() {

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

@@ -20,7 +20,7 @@ class _AnimationControllerDemoState extends State<AnimationControllerDemo>
   // Widget is not visible.
 
   static const Duration _duration = Duration(seconds: 1);
-  AnimationController controller;
+  late final AnimationController controller;
 
   @override
   void initState() {

animations/lib/src/basics/04_tweens.dart

@@ -15,8 +15,8 @@ class _TweenDemoState extends State<TweenDemo>
     with SingleTickerProviderStateMixin {
   static const Duration _duration = Duration(seconds: 1);
   static const double accountBalance = 1000000;
-  AnimationController controller;
-  Animation<double> animation;
+  late final AnimationController controller;
+  late final Animation<double> animation;
 
   @override
   void initState() {

animations/lib/src/basics/05_animated_builder.dart

@@ -16,8 +16,8 @@ class _AnimatedBuilderDemoState extends State<AnimatedBuilderDemo>
   static const Color beginColor = Colors.deepPurple;
   static const Color endColor = Colors.deepOrange;
   Duration duration = Duration(milliseconds: 800);
-  AnimationController controller;
-  Animation<Color> animation;
+  late AnimationController controller;
+  late Animation<Color?> animation;
 
   @override
   void initState() {

animations/lib/src/basics/06_custom_tween.dart

@@ -5,13 +5,13 @@
 import 'package:flutter/material.dart';
 
 class TypewriterTween extends Tween<String> {
-  TypewriterTween({String begin = '', String end})
+  TypewriterTween({String begin = '', String end = ''})
       : super(begin: begin, end: end);
 
   @override
   String lerp(double t) {
-    var cutoff = (end.length * t).round();
-    return end.substring(0, cutoff);
+    var cutoff = (end!.length * t).round();
+    return end!.substring(0, cutoff);
   }
 }
 
@@ -26,8 +26,8 @@ class _CustomTweenDemoState extends State<CustomTweenDemo>
     with SingleTickerProviderStateMixin {
   static const Duration _duration = Duration(seconds: 3);
   static const String message = loremIpsum;
-  AnimationController controller;
-  Animation<String> animation;
+  late final AnimationController controller;
+  late final Animation<String> animation;
 
   @override
   void initState() {

animations/lib/src/basics/07_tween_sequence.dart

@@ -14,8 +14,8 @@ class TweenSequenceDemo extends StatefulWidget {
 class _TweenSequenceDemoState extends State<TweenSequenceDemo>
     with SingleTickerProviderStateMixin {
   static const Duration duration = Duration(seconds: 3);
-  AnimationController controller;
-  Animation<Color> animation;
+  late final AnimationController controller;
+  late final Animation<Color?> animation;
 
   static final colors = [
     Colors.red,
@@ -31,7 +31,7 @@ class _TweenSequenceDemoState extends State<TweenSequenceDemo>
   void initState() {
     super.initState();
 
-    final sequenceItems = <TweenSequenceItem<Color>>[];
+    final sequenceItems = <TweenSequenceItem<Color?>>[];
 
     for (var i = 0; i < colors.length; i++) {
       final beginColor = colors[i];
@@ -39,7 +39,7 @@ class _TweenSequenceDemoState extends State<TweenSequenceDemo>
       final weight = 1 / colors.length;
 
       sequenceItems.add(
-        TweenSequenceItem<Color>(
+        TweenSequenceItem<Color?>(
           tween: ColorTween(begin: beginColor, end: endColor),
           weight: weight,
         ),
@@ -47,7 +47,7 @@ class _TweenSequenceDemoState extends State<TweenSequenceDemo>
     }
 
     controller = AnimationController(duration: duration, vsync: this);
-    animation = TweenSequence<Color>(sequenceItems).animate(controller);
+    animation = TweenSequence<Color?>(sequenceItems).animate(controller);
   }
 
   @override

animations/lib/src/basics/08_fade_transition.dart

@@ -15,9 +15,9 @@ class FadeTransitionDemo extends StatefulWidget {
 
 class _FadeTransitionDemoState extends State<FadeTransitionDemo>
     with SingleTickerProviderStateMixin {
-  AnimationController _controller;
-  Animation<double> _animation;
-  CurvedAnimation _curve;
+  late final AnimationController _controller;
+  late final Animation<double> _animation;
+  late final CurvedAnimation _curve;
 
   @override
   void initState() {