Address comments from #5714 (#15886)

* add simple test case for passing shadow color through physical layer

* Update physical_model_test.dart

add newline

packages/flutter/test/widgets/physical_model_test.dart

+import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
+import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('PhysicalModel - creates a physical model layer when it needs compositing', (WidgetTester tester) async {
+    await tester.pumpWidget(new Directionality(
+      textDirection: TextDirection.ltr,
+      child: new PhysicalModel(
+        shape: BoxShape.rectangle,
+        color: Colors.grey,
+        shadowColor: Colors.red,
+        elevation: 1.0,
+        child: new TextField(controller: new TextEditingController()),
+      ),
+    ));
+    await tester.pump();
+
+    final RenderPhysicalModel renderPhysicalModel = tester.allRenderObjects.firstWhere((RenderObject object) => object is RenderPhysicalModel);
+    expect(renderPhysicalModel.needsCompositing, true);
+
+    final PhysicalModelLayer physicalModelLayer = tester.layers.firstWhere((Layer layer) => layer is PhysicalModelLayer);
+    expect(physicalModelLayer.shadowColor, Colors.red);
+    expect(physicalModelLayer.color, Colors.grey);
+    expect(physicalModelLayer.elevation, 1.0);
+  });
+}