Fix and cleanup implicit animations codelab (#9201)

Essentially this PR is focused on improving the clarity of the steps
which are diff based, matching starting and ending runnable samples to
the steps, removing unnecessary code, and completing some cleanup of the
examples and the surrounding text.

**Detailed list of changes:**

- Sets up code excerpts for the starting and complete example code which
were mismatched with the actual steps/instructions. Then updates them to
match.
- Adds missing type specifiers and simplifies variable declarations.
- Adjusts trailing commas and formatting in opacity sample for easier to
read diffs.
- Removes extra folders/steps that are not currently used.
- Adds missing type specifiers and simplifies variable declarations.
- Moves away from inline ignores since the diff tooling doesn't remove
them, making the diffs more confusing and the line numbers inconsistent
between steps. Ignoring in the analysis options file allows for
consistent line numbers.
- Changes text of button from "change" to "Change" to match
instructions.
- Fixes a few instructions in the codelab's text.
- Removes the embedded survey.
- Adds a README to the example directory explaining its setup.

Closes #6929

examples/animation/implicit/README.md

@@ -0,0 +1,20 @@
+# Implicit animations codelab examples
+
+The code samples within this folder are used
+within the [Implicit animations codelab][].
+
+- `container1` -> `container5` are used for the
+  steps of "Shape-shifting effect" portion.
+- `container1` is also used for the runnable starter sample and
+  `container5` for the complete sample.
+- `container6` is an extra step that showcases specifying a
+  non-default animation curve.
+- `opacity1` -> `opacity5` are used for the
+  steps of "Fade-in text effect" portion.
+- `opacity1` is also used for the runnable starter sample and
+  `opacity5` for the complete sample.
+- The severity of `missing_required_argument` is ignored manually
+  in the [`analysis_options.yaml`](analysis_options.yaml) file since the
+  diffing infrastructure currently can't handle the `ignore` for it.
+
+[Implicit animations codelab]: https://docs.flutter.dev/codelabs/implicit-animations

examples/animation/implicit/analysis_options.yaml

@@ -3,3 +3,8 @@
 # overrides for individual lints.
 
 include: ../../analysis_options.yaml
+
+analyzer:
+  errors:
+    # Ignored since ignore_for_files can't be hidden in diff excerpts yet
+    missing_required_argument: ignore

examples/animation/implicit/container1/lib/main.dart

@@ -31,7 +31,7 @@ class _AnimatedContainerDemoState extends State<AnimatedContainerDemo> {
   late double margin;
 
   @override
-  initState() {
+  void initState() {
     super.initState();
     color = randomColor();
     borderRadius = randomBorderRadius();
@@ -56,7 +56,7 @@ class _AnimatedContainerDemoState extends State<AnimatedContainerDemo> {
               ),
             ),
             ElevatedButton(
-              child: const Text('change'),
+              child: const Text('Change'),
               onPressed: () => {},
             ),
           ],

examples/animation/implicit/container2/lib/main.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license
 // that can be found in the LICENSE file.
 
-// ignore_for_file: missing_required_argument
-
 import 'dart:math';
 
 import 'package:flutter/material.dart';
@@ -33,7 +31,7 @@ class _AnimatedContainerDemoState extends State<AnimatedContainerDemo> {
   late double margin;
 
   @override
-  initState() {
+  void initState() {
     super.initState();
     color = randomColor();
     borderRadius = randomBorderRadius();
@@ -58,7 +56,7 @@ class _AnimatedContainerDemoState extends State<AnimatedContainerDemo> {
               ),
             ),
             ElevatedButton(
-              child: const Text('change'),
+              child: const Text('Change'),
               onPressed: () => {},
             ),
           ],

examples/animation/implicit/container3/lib/main.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license
 // that can be found in the LICENSE file.
 
-// ignore_for_file: missing_required_argument
-
 import 'dart:math';
 
 import 'package:flutter/material.dart';
@@ -33,7 +31,7 @@ class _AnimatedContainerDemoState extends State<AnimatedContainerDemo> {
   late double margin;
 
   @override
-  initState() {
+  void initState() {
     super.initState();
     color = randomColor();
     borderRadius = randomBorderRadius();
@@ -66,7 +64,7 @@ class _AnimatedContainerDemoState extends State<AnimatedContainerDemo> {
               ),
             ),
             ElevatedButton(
-              child: const Text('change'),
+              child: const Text('Change'),
               onPressed: () => {},
             ),
           ],

examples/animation/implicit/container4/lib/main.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license
 // that can be found in the LICENSE file.
 
-// ignore_for_file: missing_required_argument
-
 import 'dart:math';
 
 import 'package:flutter/material.dart';
@@ -33,7 +31,7 @@ class _AnimatedContainerDemoState extends State<AnimatedContainerDemo> {
   late double margin;
 
   @override
-  initState() {
+  void initState() {
     super.initState();
     color = randomColor();
     borderRadius = randomBorderRadius();
@@ -66,7 +64,7 @@ class _AnimatedContainerDemoState extends State<AnimatedContainerDemo> {
               ),
             ),
             ElevatedButton(
-              child: const Text('change'),
+              child: const Text('Change'),
               onPressed: () => change(),
             ),
           ],

examples/animation/implicit/container5/lib/main.dart

@@ -33,7 +33,7 @@ class _AnimatedContainerDemoState extends State<AnimatedContainerDemo> {
   late double margin;
 
   @override
-  initState() {
+  void initState() {
     super.initState();
     color = randomColor();
     borderRadius = randomBorderRadius();
@@ -67,7 +67,7 @@ class _AnimatedContainerDemoState extends State<AnimatedContainerDemo> {
               ),
             ),
             ElevatedButton(
-              child: const Text('change'),
+              child: const Text('Change'),
               onPressed: () => change(),
             ),
           ],

examples/animation/implicit/container6/lib/main.dart

@@ -33,7 +33,7 @@ class _AnimatedContainerDemoState extends State<AnimatedContainerDemo> {
   late double margin;
 
   @override
-  initState() {
+  void initState() {
     super.initState();
     color = randomColor();
     borderRadius = randomBorderRadius();
@@ -68,7 +68,7 @@ class _AnimatedContainerDemoState extends State<AnimatedContainerDemo> {
               ),
             ),
             ElevatedButton(
-              child: const Text('change'),
+              child: const Text('Change'),
               onPressed: () => change(),
             ),
           ],