Don't crash if no widthFactor is set in an unbounded context

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

@@ -192,13 +192,13 @@ class RenderPositionedBox extends RenderShiftedBox {
   }
 
   void performLayout() {
-    final bool shrinkWrapWidth = widthFactor != null || constraints.maxWidth == double.INFINITY;
-    final bool shrinkWrapHeight = heightFactor != null || constraints.maxHeight == double.INFINITY;
+    final bool shrinkWrapWidth = _widthFactor != null || constraints.maxWidth == double.INFINITY;
+    final bool shrinkWrapHeight = _heightFactor != null || constraints.maxHeight == double.INFINITY;
 
     if (child != null) {
       child.layout(constraints.loosen(), parentUsesSize: true);
-      size = constraints.constrain(new Size(shrinkWrapWidth ? child.size.width * _widthFactor : double.INFINITY,
-                                            shrinkWrapHeight ? child.size.height * _heightFactor : double.INFINITY));
+      size = constraints.constrain(new Size(shrinkWrapWidth ? child.size.width * (_widthFactor ?? 1.0) : double.INFINITY,
+                                            shrinkWrapHeight ? child.size.height * (_heightFactor ?? 1.0) : double.INFINITY));
       final Offset delta = size - child.size;
       final BoxParentData childParentData = child.parentData;
       childParentData.position = delta.scale(_alignment.x, _alignment.y).toPoint();

packages/unit/test/widget/align_test.dart

+import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 import 'package:test/test.dart';
 
@@ -21,4 +22,26 @@ void main() {
       );
     });
   });
+
+  test('Shrink wraps in finite space', () {
+    testWidgets((WidgetTester tester) {
+      GlobalKey alignKey = new GlobalKey();
+      tester.pumpWidget(
+        new ScrollableViewport(
+          child: new Align(
+            key: alignKey,
+            child: new Container(
+              width: 10.0,
+              height: 10.0
+            ),
+            alignment: const FractionalOffset(0.50, 0.50)
+          )
+        )
+      );
+
+      RenderBox box = alignKey.currentContext.findRenderObject();
+      expect(box.size.width, equals(800.0));
+      expect(box.size.height, equals(10.0));
+    });
+  });
 }