Shrink wrap when constraints are infinite.

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

@@ -194,8 +194,9 @@ class RenderPositionedBox extends RenderShiftedBox {
     markNeedsLayout();
   }
 
-  bool get _shinkWrapWidth => _shrinkWrap == ShrinkWrap.width || _shrinkWrap == ShrinkWrap.both;
-  bool get _shinkWrapHeight => _shrinkWrap == ShrinkWrap.height || _shrinkWrap == ShrinkWrap.both;
+  // These are only valid during performLayout() and paint(), since they rely on constraints which is only set after layout() is called.
+  bool get _shinkWrapWidth => _shrinkWrap == ShrinkWrap.width || _shrinkWrap == ShrinkWrap.both || constraints.maxWidth == double.INFINITY;
+  bool get _shinkWrapHeight => _shrinkWrap == ShrinkWrap.height || _shrinkWrap == ShrinkWrap.both || constraints.maxHeight == double.INFINITY;
 
   void performLayout() {
     if (child != null) {

packages/unit/test/widget/center_test.dart

+import 'package:sky/widgets.dart';
+import 'package:test/test.dart';
+
+import 'widget_tester.dart';
+
+void main() {
+  test('Can be placed in an infinte box', () {
+    WidgetTester tester = new WidgetTester();
+
+    tester.pumpFrame(() {
+      return new Block([new Center()]);
+    });
+  });
+
+}