[material_3_demo] Refactor application code into multiple small libraries (#2581)

material_3_demo/lib/src/animations.dart

@@ -0,0 +1,31 @@
+// Copyright 2021 The Flutter team. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+
+class SizeAnimation extends CurvedAnimation {
+  SizeAnimation(Animation<double> parent)
+    : super(
+        parent: parent,
+        curve: const Interval(0.2, 0.8, curve: Curves.easeInOutCubicEmphasized),
+        reverseCurve: Interval(
+          0,
+          0.2,
+          curve: Curves.easeInOutCubicEmphasized.flipped,
+        ),
+      );
+}
+
+class OffsetAnimation extends CurvedAnimation {
+  OffsetAnimation(Animation<double> parent)
+    : super(
+        parent: parent,
+        curve: const Interval(0.4, 1.0, curve: Curves.easeInOutCubicEmphasized),
+        reverseCurve: Interval(
+          0,
+          0.2,
+          curve: Curves.easeInOutCubicEmphasized.flipped,
+        ),
+      );
+}