Scaffold shouldn't assert with large bottom padding (#8415)

Fixes #8413

packages/flutter/lib/src/material/scaffold.dart

@@ -48,7 +48,7 @@ class _ScaffoldLayout extends MultiChildLayoutDelegate {
 
   @override
   void performLayout(Size size) {
-    BoxConstraints looseConstraints = new BoxConstraints.loose(size);
+    final BoxConstraints looseConstraints = new BoxConstraints.loose(size);
 
     // This part of the layout has the same effect as putting the app bar and
     // body in a column and making the body flexible. What's different is that
@@ -56,8 +56,8 @@ class _ScaffoldLayout extends MultiChildLayoutDelegate {
     // so the app bar's shadow is drawn on top of the body.
 
     final BoxConstraints fullWidthConstraints = looseConstraints.tighten(width: size.width);
+    final double bottom = math.max(0.0, size.height - padding.bottom);
     double contentTop = 0.0;
-    double bottom = size.height - padding.bottom;
     double contentBottom = bottom;
 
     if (hasChild(_ScaffoldSlot.appBar)) {
@@ -78,10 +78,9 @@ class _ScaffoldLayout extends MultiChildLayoutDelegate {
     }
 
     if (hasChild(_ScaffoldSlot.body)) {
-      final double bodyHeight = contentBottom - contentTop;
       final BoxConstraints bodyConstraints = new BoxConstraints(
         maxWidth: fullWidthConstraints.maxWidth,
-        maxHeight: bodyHeight,
+        maxHeight: math.max(0.0, contentBottom - contentTop),
       );
       layoutChild(_ScaffoldSlot.body, bodyConstraints);
       positionChild(_ScaffoldSlot.body, new Offset(0.0, contentTop));

packages/flutter/test/material/scaffold_test.dart

@@ -41,6 +41,46 @@ void main() {
     expect(bodyBox.size, equals(const Size(800.0, 544.0)));
   });
 
+  testWidgets('Scaffold large bottom padding test', (WidgetTester tester) async {
+    Key bodyKey = new UniqueKey();
+    await tester.pumpWidget(new MediaQuery(
+      data: new MediaQueryData(
+        padding: const EdgeInsets.only(bottom: 700.0),
+      ),
+      child: new Scaffold(
+        body: new Container(key: bodyKey),
+      ),
+    ));
+
+    RenderBox bodyBox = tester.renderObject(find.byKey(bodyKey));
+    expect(bodyBox.size, equals(const Size(800.0, 0.0)));
+
+    await tester.pumpWidget(new MediaQuery(
+      data: new MediaQueryData(
+        padding: const EdgeInsets.only(bottom: 500.0),
+      ),
+      child: new Scaffold(
+        body: new Container(key: bodyKey),
+      ),
+    ));
+
+    expect(bodyBox.size, equals(const Size(800.0, 100.0)));
+
+    await tester.pumpWidget(new MediaQuery(
+      data: new MediaQueryData(
+        padding: const EdgeInsets.only(bottom: 580.0),
+      ),
+      child: new Scaffold(
+        appBar: new AppBar(
+          title: new Text('Title'),
+        ),
+        body: new Container(key: bodyKey),
+      ),
+    ));
+
+    expect(bodyBox.size, equals(const Size(800.0, 0.0)));
+  });
+
   testWidgets('Floating action animation', (WidgetTester tester) async {
     await tester.pumpWidget(new Scaffold(
       floatingActionButton: new FloatingActionButton(