Add missing returns.

As of `1.18.0-dev-0`, these cases will get flagged.  In the meantime, the

examples/flutter_gallery/lib/demo/calculator/logic.dart

@@ -83,6 +83,7 @@ class OperationToken extends ExpressionToken {
       case Operation.Division:
         return '  \u00F7  ';
     }
+    return null;
   }
 }
 
@@ -258,6 +259,7 @@ class CalcExpression {
       case ExpressionState.Result:
         return appendOperation(Operation.Subtraction);
     }
+    return null;
   }
 
   /// Computes the result of the current expression and returns a new

examples/flutter_gallery/lib/demo/grid_list_demo.dart

@@ -130,6 +130,7 @@ class GridDemoPhotoItem extends StatelessWidget {
           child: image
         );
     }
+    return null;
   }
 }
 

examples/stocks/lib/main.dart

@@ -75,6 +75,7 @@ class StocksAppState extends State<StocksApp> {
           accentColor: Colors.redAccent[200]
         );
     }
+    return null;
   }
 
   Route<Null> _getRoute(RouteSettings settings) {

packages/flutter/lib/src/animation/animations.dart

@@ -287,6 +287,7 @@ class ReverseAnimation extends Animation<double>
       case AnimationStatus.completed: return AnimationStatus.dismissed;
       case AnimationStatus.dismissed: return AnimationStatus.completed;
     }
+    return null;    
   }
 
   @override

packages/flutter/lib/src/cassowary/expression.dart

@@ -157,7 +157,6 @@ class Expression extends EquationMember {
     if (!m.isConstant) {
       throw new ParserException(
           'The divisor was not a constant expression', <EquationMember>[this, m]);
-      return null;
     }
 
     return this._applyMultiplicand(1.0 / m.value);

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

@@ -227,6 +227,7 @@ class _MaterialButtonState extends State<MaterialButton> {
           return Colors.white30;
       }
     }
+    return null;
   }
 
   void _handleHighlightChanged(bool value) {

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

@@ -63,6 +63,7 @@ class Dialog extends StatelessWidget {
       case Brightness.dark:
         return Colors.grey[800];
     }
+    return null;
   }
 
   @override

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

@@ -67,6 +67,7 @@ class DrawerItem extends StatelessWidget {
           return Colors.white30;
         return null; // use default icon theme colour unmodified
     }
+    return null;  
   }
 
   TextStyle _getTextStyle(ThemeData themeData) {

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

@@ -65,6 +65,7 @@ class Icon extends StatelessWidget {
       case Brightness.light:
         return Colors.black;
     }
+    return null;    
   }
 
   Color _getDefaultColor(BuildContext context) {

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

@@ -104,6 +104,7 @@ class RaisedButton extends StatelessWidget {
         case Brightness.dark:
           return Colors.white12;
       }
+      return null;
     }
   }
 

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

@@ -186,4 +186,5 @@ Widget buildTextSelectionHandle(
         child: handle
       );
   }
+  return null;
 }

packages/flutter/lib/src/painting/box_painter.dart

@@ -1353,6 +1353,7 @@ class BoxDecoration extends Decoration {
         double distance = (position - center).distance;
         return distance <= math.min(size.width, size.height) / 2.0;
     }
+    return null;
   }
 
   @override

packages/flutter/lib/src/painting/text_painter.dart

@@ -125,6 +125,7 @@ class TextPainter {
       case TextBaseline.ideographic:
         return _paragraph.ideographicBaseline;
     }
+    return null;
   }
 
   double _lastMinWidth;
@@ -211,6 +212,7 @@ class TextPainter {
             ?? _getOffsetFromUpstream(offset, caretPrototype)
             ?? emptyOffset;
     }
+    return null;
   }
 
   /// Returns a list of rects that bound the given selection.

packages/flutter/lib/src/rendering/box.dart

@@ -1125,7 +1125,6 @@ abstract class RenderBox extends RenderObject {
           'set a size and lay out any children, or, set sizedByParent to true '
           'so that performResize() sizes the render object.'
         );
-        return true;
       }
       return true;
     });

packages/flutter/lib/src/rendering/list.dart

@@ -82,6 +82,7 @@ class RenderList extends RenderVirtualViewport<ListParentData> {
       case Axis.horizontal:
         return padding.horizontal;
     }
+    return null;
   }
 
   double get _preferredExtent {

packages/flutter/lib/src/rendering/viewport.dart

@@ -57,6 +57,7 @@ class ViewportDimensions {
       case ViewportAnchor.end:
         return paintOffset + (containerSize - contentSize);
     }
+    return null;
   }
 
   @override

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

@@ -112,6 +112,7 @@ class BannerPainter extends CustomPainter {
       case BannerLocation.topLeft:
         return -math.PI / 4.0;
     }
+    return null;
   }
 }
 

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

@@ -245,7 +245,6 @@ class _DismissableState extends State<Dismissable> {
           return vy > _kMinFlingVelocity;
       }
     }
-    return false;
   }
 
   void _handleDragEnd(DragEndDetails details) {

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

@@ -740,6 +740,7 @@ class _LazyBlockElement extends RenderObjectElement {
       case Axis.vertical:
         return new Offset(0.0, size.height);
     }
+    return null;    
   }
 
   static RenderBox _getNextWithin(_RenderLazyBlock block, RenderBox child) {

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

@@ -224,6 +224,7 @@ abstract class PageableState<T extends Pageable> extends ScrollableState<T> {
       case Axis.vertical:
         return box.size.height;
     }
+    return null;
   }
 
   @override

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

@@ -285,6 +285,7 @@ class ScrollableState<T extends Scrollable> extends State<T> {
       case ViewportAnchor.end:
         return pixelOffset;
     }
+    return null;
   }
 
   /// Convert a scrollOffset value to the number of pixels to which it corresponds.

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

@@ -239,6 +239,7 @@ class _VirtualListViewport extends VirtualViewport {
         }
         break;
     }
+    return null;
   }
 
   @override

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

@@ -294,5 +294,6 @@ class _TextSelectionHandleOverlayState extends State<_TextSelectionHandleOverlay
       case TextDirection.rtl:
         return rtlType;
     }
+    return null;
   }
 }

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

@@ -70,6 +70,7 @@ abstract class VirtualViewportElement extends RenderObjectElement {
       case ViewportAnchor.end:
         return scrollOffset;
     }
+    return null;    
   }
 
   /// Returns a two-dimensional representation of the scroll offset, accounting

packages/flutter/test/widget/render_object_widget_test.dart

@@ -29,6 +29,7 @@ class TestOrientedBox extends SingleChildRenderObjectWidget {
       case Orientation.portrait:
         return new BoxDecoration(backgroundColor: const Color(0xFF0000FF));
     }
+    return null;
   }
 
   @override

packages/flutter_tools/lib/src/application_package.dart

@@ -134,6 +134,7 @@ ApplicationPackage getApplicationPackageForPlatform(TargetPlatform platform) {
     case TargetPlatform.linux_x64:
       return null;
   }
+  return null;
 }
 
 class ApplicationPackageStore {
@@ -156,5 +157,6 @@ class ApplicationPackageStore {
       case TargetPlatform.linux_x64:
         return null;
     }
+    return null;
   }
 }

packages/flutter_tools/lib/src/build_info.dart

@@ -53,6 +53,7 @@ String getNameForHostPlatform(HostPlatform platform) {
       return 'linux-x64';
   }
   assert(false);
+  return null;
 }
 
 enum TargetPlatform {
@@ -80,6 +81,7 @@ String getNameForTargetPlatform(TargetPlatform platform) {
       return 'linux-x64';
   }
   assert(false);
+  return null;
 }
 
 TargetPlatform getTargetPlatformForName(String platform) {

packages/flutter_tools/lib/src/ios/devices.dart

@@ -235,7 +235,6 @@ class IOSDevice extends Device {
     } else {
       return false;
     }
-    return false;
   }
 
   @override