Add FlutterLogoDecoration tests (#10209)

Covers FlutterLogoDecoration.lerp()

Disabling const constructor lint for the test values pending resolution of
https://github.com/dart-lang/sdk/issues/26980

packages/flutter/test/painting/flutter_logo_test.dart

+// Copyright 2017 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/painting.dart';
+
+void main() {
+  // Here and below, see: https://github.com/dart-lang/sdk/issues/26980
+  // ignore: prefer_const_constructors
+  final FlutterLogoDecoration start = new FlutterLogoDecoration(
+    lightColor: const Color(0xFF000000),
+    darkColor: const Color(0xFFFFFFFF),
+    textColor: const Color(0xFFD4F144),
+    style: FlutterLogoStyle.stacked,
+    margin: const EdgeInsets.all(10.0),
+  );
+
+  // ignore: prefer_const_constructors
+  final FlutterLogoDecoration end = new FlutterLogoDecoration(
+    lightColor: const Color(0xFFFFFFFF),
+    darkColor: const Color(0xFF000000),
+    textColor: const Color(0xFF81D4FA),
+    style: FlutterLogoStyle.stacked,
+    margin: const EdgeInsets.all(10.0),
+  );
+
+  test('FlutterLogoDecoration lerp from null to null is null', () {
+    final FlutterLogoDecoration logo = FlutterLogoDecoration.lerp(null, null, 0.5);
+    expect(logo, isNull);
+  });
+
+  test('FlutterLogoDecoration lerp from non-null to null lerps margin', () {
+    final FlutterLogoDecoration logo = FlutterLogoDecoration.lerp(start, null, 0.4);
+    expect(logo.lightColor, start.lightColor);
+    expect(logo.darkColor, start.darkColor);
+    expect(logo.textColor, start.textColor);
+    expect(logo.style, start.style);
+    expect(logo.margin, start.margin * 0.4);
+  });
+
+  test('FlutterLogoDecoration lerp from null to non-null lerps margin', () {
+    final FlutterLogoDecoration logo = FlutterLogoDecoration.lerp(null, end, 0.6);
+    expect(logo.lightColor, end.lightColor);
+    expect(logo.darkColor, end.darkColor);
+    expect(logo.textColor, end.textColor);
+    expect(logo.style, end.style);
+    expect(logo.margin, end.margin * 0.6);
+  });
+
+  test('FlutterLogoDecoration lerps colors and margins', () {
+    final FlutterLogoDecoration logo = FlutterLogoDecoration.lerp(start, end, 0.5);
+    expect(logo.lightColor, Color.lerp(start.lightColor, end.lightColor, 0.5));
+    expect(logo.darkColor, Color.lerp(start.darkColor, end.darkColor, 0.5));
+    expect(logo.textColor, Color.lerp(start.textColor, end.textColor, 0.5));
+    expect(logo.margin, EdgeInsets.lerp(start.margin, end.margin, 0.5));
+  });
+
+  test('FlutterLogoDecoration lerp changes styles at 0.5', () {
+    FlutterLogoDecoration logo = FlutterLogoDecoration.lerp(start, end, 0.4);
+    expect(logo.style, start.style);
+
+    logo = FlutterLogoDecoration.lerp(start, end, 0.5);
+    expect(logo.style, end.style);
+  });
+}