Fix analyzer warnings in examples

examples/rendering/flex.dart

@@ -18,7 +18,8 @@ RenderBox buildFlexExample() {
   void addFlexChildSolidColor(RenderFlex parent, ui.Color backgroundColor, { int flex: 0 }) {
     RenderSolidColorBox child = new RenderSolidColorBox(backgroundColor);
     parent.add(child);
-    child.parentData.flex = flex;
+    final FlexParentData childParentData = child.parentData;
+    childParentData.flex = flex;
   }
 
   // Yellow bar at top

examples/rendering/interactive_flex.dart

@@ -49,7 +49,8 @@ void main() {
   void addFlexChildSolidColor(RenderFlex parent, Color backgroundColor, { int flex: 0 }) {
     RenderSolidColorBox child = new RenderSolidColorBox(backgroundColor);
     parent.add(child);
-    child.parentData.flex = flex;
+    final FlexParentData childParentData = child.parentData;
+    childParentData.flex = flex;
   }
 
   var row = new RenderFlex(direction: FlexDirection.horizontal);
@@ -90,7 +91,8 @@ Pancetta meatball tongue tenderloin rump tail jowl boudin.""";
   addFlexChildSolidColor(column, const Color(0xFF0081C6), flex: 2);
 
   row.add(column);
-  column.parentData.flex = 8;
+  final FlexParentData childParentData = column.parentData;
+  childParentData.flex = 8;
 
   RenderDecoratedBox root = new RenderDecoratedBox(
     decoration: new BoxDecoration(backgroundColor: const Color(0xFFFFFFFF)),

examples/rendering/render_paragraph.dart

@@ -16,7 +16,8 @@ void main() {
 
   RenderObject child = new RenderSolidColorBox(const Color(0xFFFFFF00));
   flexRoot.add(child);
-  child.parentData.flex = 2;
+  FlexParentData childParentData = child.parentData;
+  childParentData.flex = 2;
 
   // The internet is a beautiful place.  https://baconipsum.com/
   String meatyString = """Bacon ipsum dolor amet ham fatback tri-tip, prosciutto
@@ -33,7 +34,8 @@ Pancetta meatball tongue tenderloin rump tail jowl boudin.""";
     child: new RenderParagraph(text)
   );
   flexRoot.add(child);
-  child.parentData.flex = 1;
+  childParentData = child.parentData;
+  childParentData.flex = 1;
 
   new FlutterBinding(root: root);
 }

examples/rendering/spinning_flex.dart

@@ -18,7 +18,8 @@ void main() {
   void addFlexChildSolidColor(RenderFlex parent, ui.Color backgroundColor, { int flex: 0 }) {
     RenderSolidColorBox child = new RenderSolidColorBox(backgroundColor);
     parent.add(child);
-    child.parentData.flex = flex;
+    final FlexParentData childParentData = child.parentData;
+    childParentData.flex = flex;
   }
 
   addFlexChildSolidColor(flexRoot, const ui.Color(0xFFFF00FF), flex: 1);