Add RTL support to Stepper (#11873)

Fixes #11379

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

@@ -354,7 +354,7 @@ class _StepperState extends State<Stepper> with TickerProviderStateMixin {
               child: const Text('CONTINUE'),
             ),
             new Container(
-              margin: const EdgeInsets.only(left: 8.0),
+              margin: const EdgeInsetsDirectional.only(start: 8.0),
               child: new FlatButton(
                 onPressed: widget.onStepCancel,
                 textColor: cancelColor,
@@ -457,9 +457,7 @@ class _StepperState extends State<Stepper> with TickerProviderStateMixin {
             ]
           ),
           new Container(
-            margin: const EdgeInsets.only(
-              left: 12.0
-            ),
+            margin: const EdgeInsetsDirectional.only(start: 12.0),
             child: _buildHeaderText(index)
           )
         ]
@@ -470,8 +468,8 @@ class _StepperState extends State<Stepper> with TickerProviderStateMixin {
   Widget _buildVerticalBody(int index) {
     return new Stack(
       children: <Widget>[
-        new Positioned(
-          left: 24.0,
+        new PositionedDirectional(
+          start: 24.0,
           top: 0.0,
           bottom: 0.0,
           child: new SizedBox(
@@ -489,9 +487,9 @@ class _StepperState extends State<Stepper> with TickerProviderStateMixin {
         new AnimatedCrossFade(
           firstChild: new Container(height: 0.0),
           secondChild: new Container(
-            margin: const EdgeInsets.only(
-              left: 60.0,
-              right: 24.0,
+            margin: const EdgeInsetsDirectional.only(
+              start: 60.0,
+              end: 24.0,
               bottom: 24.0,
             ),
             child: new Column(
@@ -565,7 +563,7 @@ class _StepperState extends State<Stepper> with TickerProviderStateMixin {
                 ),
               ),
               new Container(
-                margin: const EdgeInsets.only(left: 12.0),
+                margin: const EdgeInsetsDirectional.only(start: 12.0),
                 child: _buildHeaderText(i),
               ),
             ],