We should lerp decorations in the correct direction (#3176)

Previously we lerped them backwards.



Fixes #2832

packages/flutter/lib/src/painting/decoration.dart

@@ -51,9 +51,9 @@ abstract class Decoration {
   /// otherwise it uses [begin]'s [lerpFrom] function.
   static Decoration lerp(Decoration begin, Decoration end, double t) {
     if (end != null)
-      return end.lerpTo(begin, t);
+      return end.lerpFrom(begin, t);
     if (begin != null)
-      return begin.lerpFrom(end, t);
+      return begin.lerpTo(end, t);
     return null;
   }
 

packages/flutter/test/painting/decoration_test.dart

+// Copyright 2016 The Chromium Authors. 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/painting.dart';
+
+import 'package:test/test.dart';
+
+void main() {
+  test("Decoration.lerp()", () {
+    BoxDecoration a = new BoxDecoration(backgroundColor: const Color(0xFFFFFFFF));
+    BoxDecoration b = new BoxDecoration(backgroundColor: const Color(0x00000000));
+
+    BoxDecoration c = Decoration.lerp(a, b, 0.0);
+    expect(c.backgroundColor, equals(a.backgroundColor));
+
+    c = Decoration.lerp(a, b, 0.25);
+    expect(c.backgroundColor, equals(Color.lerp(const Color(0xFFFFFFFF), const Color(0x00000000), 0.25)));
+
+    c = Decoration.lerp(a, b, 1.0);
+    expect(c.backgroundColor, equals(b.backgroundColor));
+  });
+}