RenderOffStage test

Also, give RenderViewport a default offset.

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

@@ -32,7 +32,7 @@ class RenderViewport extends RenderBox with RenderObjectWithChildMixin<RenderBox
 
   RenderViewport({
     RenderBox child,
-    Offset scrollOffset,
+    Offset scrollOffset: Offset.zero,
     ScrollDirection scrollDirection: ScrollDirection.vertical
   }) : _scrollOffset = scrollOffset,
        _scrollDirection = scrollDirection {

packages/unit/test/rendering/offstage_test.dart

@@ -0,0 +1,32 @@
+import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
+import 'package:test/test.dart';
+
+import 'rendering_tester.dart';
+
+void main() {
+  test("offstage", () {
+    RenderBox child;
+    bool painted = false;
+    // viewport incoming constraints are tight 800x600
+    // viewport is vertical by default
+    RenderBox root = new RenderViewport(
+      child: new RenderOffStage(
+        child: new RenderCustomPaint(
+          child: child = new RenderConstrainedBox(
+            additionalConstraints: new BoxConstraints.tightFor(height: 10.0, width: 10.0)
+          ),
+          onPaint: (PaintingCanvas canvas, Size size) {
+            painted = true;
+          }
+        )
+      )
+    );
+    expect(child.hasSize, isFalse);
+    expect(painted, isFalse);
+    layout(root, phase: EnginePhase.paint);
+    expect(child.hasSize, isTrue);
+    expect(painted, isFalse);
+    expect(child.size, equals(const Size(800.0, 10.0)));
+  });
+}