Repository: flutter
Issue: Scrolling content is shown above BoxShadow instead of below it
Labels: framework,a: fidelity,f: scrolling,has reproducible steps,P2,found in release: 3.3,found in release: 3.6,team-framework,triaged-framework

For some reason `BoxShadow` it is not being shown correctly when scrollable content need to pass below it. 
Don't know hoy to explain this so here it is a full example code to reproduce the behavior/bug and two pictures:

```dart
import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Bug demo',
      theme: new ThemeData(
        primaryColor: Colors.white,
      ),
      home: Home(),
    );
  }
}

class Home extends StatefulWidget {
  @override
  HomeState createState() => new HomeState();
}

class HomeState extends State<Home> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.white,
      body: SafeArea(
        child: Container(
          child: Column(
            children: <Widget>[
              Row(
                children: <Widget>[
                  Expanded(
                    child: Container(
                      decoration: BoxDecoration(
                        color: Colors.white,
                        boxShadow: <BoxShadow>[
                          BoxShadow(
                            color: Colors.black26,
                            offset: Offset(0.0, 8.0),
                            blurRadius: 2.0,
                            spreadRadius: 0.0,
                          ),
                        ],
                      ),
                      padding: const EdgeInsets.only(top: 8.0, bottom: 8.0),
                      child: Container(),
                    ),
                  ),
                ],
              ),
              Expanded(
                child: Container(
                  child: SingleChildScrollView(
                    padding: EdgeInsets.all(20.0),
                    child: Container(
                      child: Wrap(
                        spacing: 0.0, // gap between adjacent chips
                        runSpacing: 12.0, // gap between lines
                        children: linesArray(),
                      ),
                    ),
                  ),
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }

  List<Widget> linesArray() {
    List<Widget> lines = List<Widget>();
    for (var i = 0; i < 10; i++) {
      lines.add(
        Row(
          children: <Widget>[
            Expanded(
              child: Container(
                height: 100.0,
                color: Colors.yellow,
              ),
            ),
          ],
        ),
      );
    }
    return lines;
  }
}

```
EDIT: This is happening both on iOS and Android :)

| Header | Header |
|--------|--------|
| ![simulator screen shot - iphone x - 2018-10-11 at 10 32 22](https://user-images.githubusercontent.com/14978705/46807779-2ad57280-cd41-11e8-9ab2-8f3eee5fe310.png) | ![simulator screen shot - iphone x - 2018-10-11 at 10 32 25](https://user-images.githubusercontent.com/14978705/46807796-36289e00-cd41-11e8-93cd-9403e803cdb8.png) |