Add some tests for these tweens (#4804)

packages/flutter/test/widget/implicit_animations_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_test/flutter_test.dart';
+import 'package:flutter/widgets.dart';
+
+void main() {
+  testWidgets('BoxConstraintsTween control test', (WidgetTester tester) async {
+    BoxConstraintsTween tween = new BoxConstraintsTween(
+      begin: new BoxConstraints.tight(new Size(20.0, 50.0)),
+      end: new BoxConstraints.tight(new Size(10.0, 30.0))
+    );
+    BoxConstraints result = tween.lerp(0.25);
+    expect(result.minWidth, 17.5);
+    expect(result.maxWidth, 17.5);
+    expect(result.minHeight, 45.0);
+    expect(result.maxHeight, 45.0);
+  });
+
+  testWidgets('DecorationTween control test', (WidgetTester tester) async {
+    DecorationTween tween = new DecorationTween(
+      begin: new BoxDecoration(backgroundColor: new Color(0xFF00FF00)),
+      end: new BoxDecoration(backgroundColor: new Color(0xFFFFFF00))
+    );
+    BoxDecoration result = tween.lerp(0.25);
+    expect(result.backgroundColor, new Color(0xFF3FFF00));
+  });
+
+  testWidgets('EdgeInsetsTween control test', (WidgetTester tester) async {
+    EdgeInsetsTween tween = new EdgeInsetsTween(
+      begin: new EdgeInsets.symmetric(vertical: 50.0),
+      end: new EdgeInsets.only(top: 10.0, bottom: 30.0)
+    );
+    EdgeInsets result = tween.lerp(0.25);
+    expect(result.left, 0.0);
+    expect(result.right, 0.0);
+    expect(result.top, 40.0);
+    expect(result.bottom, 45.0);
+  });
+
+  testWidgets('Matrix4Tween control test', (WidgetTester tester) async {
+    Matrix4Tween tween = new Matrix4Tween(
+      begin: new Matrix4.translationValues(10.0, 20.0, 30.0),
+      end: new Matrix4.translationValues(14.0, 24.0, 34.0)
+    );
+    Matrix4 result = tween.lerp(0.25);
+    expect(result, equals(new Matrix4.translationValues(11.0, 21.0, 31.0)));
+  });
+}