Added 'physics' prop to Stepper. (#26297)

* Update stepper.dart

Added 'physics' prop which allows the developer to assign the Stepper's scroll view's 'ScrollPhysics' which can solve with unwanted scrolling behaviour if the parent is also a scroll view. Defaults to 'AlwaysScrollableScrollPhysics' if null.

* Update stepper.dart

Removed unnecessary import and defaulting to 'AlwaysScrollableScrollPhysics' if physics prop is null.

* Update stepper.dart

Ran through format to remove unwanted whitespace which Analysis isn't happy about.

* Update stepper.dart

Tried reformatting again.

* Update stepper.dart

Tried reformatting again.

* Update stepper.dart

Formatting again why do you hate me github editor.

* Update stepper.dart

* Update stepper.dart

if this format doesn't work i'll cry

* Update stepper_test.dart

Added Stepper scroll tests. One that fails to find Text after Stepper if physics left as null and one that succeeds if physics set to `ClampingScrollPhysics()`

* Update stepper_test.dart

Added const constructors

* Update stepper_test.dart

trying to get rid of whitespace again

* Update stepper_test.dart

* Update stepper_test.dart

why whitespace why

* Update stepper_test.dart

* Update stepper_test.dart

* Update stepper_test.dart

* Update stepper_test.dart

Swapped to `findsNothing` because I'm an idiot.

* Update stepper_test.dart

* Update stepper.dart

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

@@ -135,6 +135,7 @@ class Stepper extends StatefulWidget {
   Stepper({
     Key key,
     @required this.steps,
+    this.physics,
     this.type = StepperType.vertical,
     this.currentStep = 0,
     this.onStepTapped,
@@ -152,6 +153,15 @@ class Stepper extends StatefulWidget {
   /// The length of [steps] must not change.
   final List<Step> steps;
 
+  /// How the stepper's scroll view should respond to user input.
+  ///
+  /// For example, determines how the scroll view continues to
+  /// animate after the user stops dragging the scroll view.
+  ///
+  /// If the stepper is contained within another scrollable it
+  /// can be helpful to set this property to [ClampingScrollPhysics].
+  final ScrollPhysics physics;
+
   /// The type of stepper that determines the layout. In the case of
   /// [StepperType.horizontal], the content of the current step is displayed
   /// underneath as opposed to the [StepperType.vertical] case where it is
@@ -594,6 +604,7 @@ class _StepperState extends State<Stepper> with TickerProviderStateMixin {
 
     return ListView(
       shrinkWrap: true,
+      physics: widget.physics,
       children: children,
     );
   }

packages/flutter/test/material/stepper_test.dart

@@ -507,4 +507,37 @@ void main() {
     renderObject = tester.renderObject(find.byIcon(Icons.check));
     expect(renderObject.size, equals(const Size.square(18.0)));
   });
+
+  testWidgets('Stepper physics scroll error test', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Material(
+          child: ListView(
+            children: <Widget>[
+              Stepper(
+                steps: const <Step>[
+                  Step(title: Text('Step 1'), content: Text('Text 1')),
+                  Step(title: Text('Step 2'), content: Text('Text 2')),
+                  Step(title: Text('Step 3'), content: Text('Text 3')),
+                  Step(title: Text('Step 4'), content: Text('Text 4')),
+                  Step(title: Text('Step 5'), content: Text('Text 5')),
+                  Step(title: Text('Step 6'), content: Text('Text 6')),
+                  Step(title: Text('Step 7'), content: Text('Text 7')),
+                  Step(title: Text('Step 8'), content: Text('Text 8')),
+                  Step(title: Text('Step 9'), content: Text('Text 9')),
+                  Step(title: Text('Step 10'), content: Text('Text 10')),
+                ],
+              ),
+              const Text('Text After Stepper'),
+            ],
+          ),
+        ),
+      ),
+    );
+
+    await tester.fling(find.byType(Stepper), const Offset(0.0, -100.0), 1000.0);
+    await tester.pumpAndSettle();
+
+    expect(find.text('Text After Stepper'), findsNothing);
+  });
 }