[CanvasKit] Allow the color list for Vertices.raw to be null

lib/web_ui/lib/src/engine/compositor/vertices.dart

@@ -6,6 +6,9 @@
 part of engine;
 
 js.JsArray<Float32List> _encodeRawColorList(Int32List rawColors) {
+  if (rawColors == null) {
+    return null;
+  }
   final int colorCount = rawColors.length;
   final List<ui.Color> colors = List<ui.Color>(colorCount);
   for (int i = 0; i < colorCount; ++i) {
@@ -36,7 +39,8 @@ class SkVertices implements ui.Vertices {
       throw ArgumentError(
           '"indices" values must be valid indices in the positions list.');
 
-    final js.JsArray<js.JsArray<double>> encodedPositions = encodePointList(positions);
+    final js.JsArray<js.JsArray<double>> encodedPositions =
+        encodePointList(positions);
     final js.JsArray<js.JsArray<double>> encodedTextures =
         encodePointList(textureCoordinates);
     final js.JsArray<Float32List> encodedColors =

lib/web_ui/test/canvaskit/vertices_test.dart

@@ -0,0 +1,32 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// @dart = 2.6
+import 'dart:typed_data';
+
+import 'package:test/test.dart';
+
+import 'package:ui/src/engine.dart';
+import 'package:ui/ui.dart' as ui;
+
+void main() {
+  group(
+    'SkVertices',
+    () {
+      setUpAll(() async {
+        await ui.webOnlyInitializePlatform();
+      });
+
+      test('Using CanvasKit', () {
+        expect(experimentalUseSkia, true);
+      });
+
+      test('Can create SkVertices.raw with null colors', () {
+        SkVertices vertices =
+            SkVertices.raw(ui.VertexMode.triangles, Float32List.fromList([]));
+        expect(vertices, isNotNull);
+      });
+    },
+  );
+}