onPressed not called when in a Positioned Flex

We were setting the main axis extent to zero because we had a sign error.

Fixes #918

packages/flutter/lib/rendering/flex.dart

@@ -420,16 +420,16 @@ class RenderFlex extends RenderBox with ContainerRenderObjectMixin<RenderBox, Fl
       betweenSpace = 0.0;
       switch (_direction) {
         case FlexDirection.horizontal:
-          size = constraints.constrain(new Size(-_overflow, crossSize));
+          size = constraints.constrain(new Size(_overflow, crossSize));
           crossSize = size.height;
-          assert(size.width >= -_overflow);
-          remainingSpace = size.width - -_overflow;
+          assert(size.width >= _overflow);
+          remainingSpace = size.width - _overflow;
           break;
         case FlexDirection.vertical:
-          size = constraints.constrain(new Size(crossSize, -_overflow));
+          size = constraints.constrain(new Size(crossSize, _overflow));
           crossSize = size.width;
-          assert(size.height >= -_overflow);
-          remainingSpace = size.height - -_overflow;
+          assert(size.height >= _overflow);
+          remainingSpace = size.height - _overflow;
           break;
       }
       _overflow = 0.0;

packages/unit/test/widget/flex_test.dart

+import 'package:sky/widgets.dart';
+import 'package:test/test.dart';
+
+import 'widget_tester.dart';
+
+void main() {
+  test('Can hit test flex children of stacks', () {
+    WidgetTester tester = new WidgetTester();
+
+    bool didReceiveTap = false;
+    tester.pumpFrame(() {
+      return new Container(
+        decoration: const BoxDecoration(
+          backgroundColor: const Color(0xFF00FF00)
+        ),
+        child: new Stack([
+          new Positioned(
+            top: 10.0,
+            left: 10.0,
+            child: new Column([
+              new GestureDetector(
+                onTap: () {
+                  didReceiveTap = true;
+                },
+                child: new Container(
+                  decoration: const BoxDecoration(
+                    backgroundColor: const Color(0xFF0000FF)
+                  ),
+                  width: 100.0,
+                  height: 100.0,
+                  child: new Center(
+                    child: new Text('X')
+                  )
+                )
+              )
+            ])
+          )
+        ])
+      );
+    });
+
+    tester.tap(tester.findText('X'));
+    expect(didReceiveTap, isTrue);
+  });
+}