Updated ColorScheme.dark() colors to match the Material Dark theme specification (#36106)

Updated ColorScheme.dark() primaryVariant, surface, background and error
colors to match the spec.

packages/flutter/lib/src/material/color_scheme.dart

@@ -76,16 +76,16 @@ class ColorScheme extends Diagnosticable {
        assert(onError != null),
        assert(brightness != null);
 
-  /// Create dark version of the
-  /// [baseline Material color scheme](https://material.io/design/color/the-color-system.html#color-theme-creation).
+  /// Create the recommended dark color scheme that matches the
+  /// [baseline Material color scheme](https://material.io/design/color/dark-theme.html#ui-application).
   const ColorScheme.dark({
     this.primary = const Color(0xffbb86fc),
-    this.primaryVariant = const Color(0xff4b01d0),
+    this.primaryVariant = const Color(0xff3700B3),
     this.secondary = const Color(0xff03dac6),
     this.secondaryVariant = const Color(0xff03dac6),
-    this.surface = Colors.black,
-    this.background = Colors.black,
-    this.error = const Color(0xffb00020),
+    this.surface = const Color(0xff121212),
+    this.background = const Color(0xff121212),
+    this.error = const Color(0xffcf6679),
     this.onPrimary = Colors.black,
     this.onSecondary = Colors.black,
     this.onSurface = Colors.white,

packages/flutter/test/material/color_scheme_test.dart

+// Copyright 2019 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/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+
+  test('light scheme matches the spec', () {
+    // Colors should match the The Material Design baseline default theme:
+    // https://material.io/design/color/dark-theme.html#ui-application
+    const ColorScheme scheme = ColorScheme.light();
+    expect(scheme.primary, const Color(0xff6200ee));
+    expect(scheme.primaryVariant, const Color(0xff3700b3));
+    expect(scheme.secondary, const Color(0xff03dac6));
+    expect(scheme.secondaryVariant, const Color(0xff018786));
+    expect(scheme.background, const Color(0xffffffff));
+    expect(scheme.surface, const Color(0xffffffff));
+    expect(scheme.error, const Color(0xffb00020));
+    expect(scheme.onPrimary, const Color(0xffffffff));
+    expect(scheme.onSecondary, const Color(0xff000000));
+    expect(scheme.onBackground, const Color(0xff000000));
+    expect(scheme.onSurface, const Color(0xff000000));
+    expect(scheme.onError, const Color(0xffffffff));
+    expect(scheme.brightness, Brightness.light);
+  });
+
+  test('dark scheme matches the spec', () {
+    // Colors should match the The Material Design baseline dark theme:
+    // https://material.io/design/color/dark-theme.html#ui-application
+    const ColorScheme scheme = ColorScheme.dark();
+    expect(scheme.primary, const Color(0xffbb86fc));
+    expect(scheme.primaryVariant, const Color(0xff3700b3));
+    expect(scheme.secondary, const Color(0xff03dac6));
+    expect(scheme.secondaryVariant, const Color(0xff03dac6));
+    expect(scheme.background, const Color(0xff121212));
+    expect(scheme.surface, const Color(0xff121212));
+    expect(scheme.error, const Color(0xffcf6679));
+    expect(scheme.onPrimary, const Color(0xff000000));
+    expect(scheme.onSecondary, const Color(0xff000000));
+    expect(scheme.onBackground, const Color(0xffffffff));
+    expect(scheme.onSurface, const Color(0xffffffff));
+    expect(scheme.onError, const Color(0xff000000));
+    expect(scheme.brightness, Brightness.dark);
+  });
+
+}
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