fix doc-comment snippets (#25579)

dev/bots/test/analyze-sample-code-test-input/known_broken_documentation.dart

@@ -39,5 +39,5 @@
 /// new Opacity(
 ///   opacity: _visible ? 1.0 : 0.0,
 ///   child: const Text('Poor wandering ones!'),
-/// ),
+/// )
 /// ```

examples/catalog/lib/custom_a11y_traversal.dart

@@ -59,7 +59,7 @@ class RowColumnTraversal extends StatelessWidget {
   /// This function expresses the sort keys as a hierarchy, but it could just as
   /// easily have been a flat list:
   ///
-  /// ```
+  /// ```dart
   /// Widget build(BuildContext context) {
   ///   return new Semantics(
   ///     sortOrder: new SemanticsSortOrder(

packages/flutter/lib/src/material/button_theme.dart

@@ -265,6 +265,7 @@ class ButtonThemeData extends Diagnosticable {
 
   /// Simply a convenience that returns [minWidth] and [height] as a
   /// [BoxConstraints] object:
+  ///
   /// ```dart
   /// return BoxConstraints(
   ///   minWidth: minWidth,

packages/flutter/lib/src/material/icon_button.dart

@@ -151,7 +151,7 @@ class IconButton extends StatelessWidget {
   ///   color: Colors.blue,
   ///   onPressed: _handleTap,
   ///   icon: Icons.widgets,
-  /// ),
+  /// )
   /// ```
   final Color color;
 

packages/flutter/lib/src/material/typography.dart

@@ -68,7 +68,8 @@ enum ScriptCategory {
 /// `Typography(platform: platform)` which uses [englishLike2014],
 /// [dense2014] and [tall2014]. To use the 2018 text theme
 /// geometries, specify a typography value:
-/// ```
+///
+/// ```dart
 /// Typography(
 ///   platorm: platform,
 ///   englishLike: Typography.englishLike2018,

packages/flutter/lib/src/widgets/icon.dart

@@ -82,7 +82,7 @@ class Icon extends StatelessWidget {
   /// Icon(
   ///   icon: Icons.widgets,
   ///   color: Colors.blue.shade400,
-  ///  ),
+  /// )
   /// ```
   final Color color;
 

packages/flutter/lib/src/widgets/platform_view.dart

@@ -117,6 +117,7 @@ class AndroidView extends StatefulWidget {
   ///
   /// For example, with the following setup vertical drags will not be dispatched to the Android
   /// view as the vertical drag gesture is claimed by the parent [GestureDetector].
+  ///
   /// ```dart
   /// GestureDetector(
   ///   onVerticalDragStart: (DragStartDetails d) {},
@@ -125,8 +126,10 @@ class AndroidView extends StatefulWidget {
   ///   ),
   /// )
   /// ```
+  ///
   /// To get the [AndroidView] to claim the vertical drag gestures we can pass a vertical drag
   /// gesture recognizer factory in [gestureRecognizers] e.g:
+  ///
   /// ```dart
   /// GestureDetector(
   ///   onVerticalDragStart: (DragStartDetails details) {},
@@ -247,6 +250,7 @@ class UiKitView extends StatefulWidget {
   ///
   /// For example, with the following setup vertical drags will not be dispatched to the UIKit
   /// view as the vertical drag gesture is claimed by the parent [GestureDetector].
+  ///
   /// ```dart
   /// GestureDetector(
   ///   onVerticalDragStart: (DragStartDetails details) {},
@@ -255,8 +259,10 @@ class UiKitView extends StatefulWidget {
   ///   ),
   /// )
   /// ```
+  ///
   /// To get the [UiKitView] to claim the vertical drag gestures we can pass a vertical drag
   /// gesture recognizer factory in [gestureRecognizers] e.g:
+  ///
   /// ```dart
   /// GestureDetector(
   ///   onVerticalDragStart: (DragStartDetails details) {},

packages/flutter/lib/src/widgets/scroll_physics.dart

@@ -41,6 +41,7 @@ class ScrollPhysics {
   /// ScrollPhysics that has [ancestor] as its parent.
   ///
   /// This method is typically used to define [applyTo] methods like:
+  ///
   /// ```dart
   /// FooScrollPhysics applyTo(ScrollPhysics ancestor) {
   ///   return FooScrollPhysics(parent: buildParent(ancestor));

packages/flutter/lib/src/widgets/text.dart

@@ -337,7 +337,6 @@ class Text extends StatelessWidget {
   ///
   /// ```dart
   /// Text(r'$$', semanticsLabel: 'Double dollars')
-  ///
   /// ```
   final String semanticsLabel;
 

packages/flutter/test/widgets/semantics_tester.dart

@@ -495,6 +495,7 @@ class SemanticsTester {
   ///   ));
   ///   semantics.dispose();
   /// });
+  /// ```
   ///
   /// At this point the unit test should automatically pass because it was
   /// generated from the actual [SemanticsNode]s. Next time the semantics tree