Canvas.concat takes a 16-element Float32List instead of an array.

sky/engine/core/painting/Matrix4.dart

Index: sky/engine/core/painting/Matrix4.dart
diff --git a/sky/engine/core/painting/Matrix4.dart b/sky/engine/core/painting/Matrix4.dart
new file mode 100644
index 0000000000000000000000000000000000000000..ba210e91728714fbc82c740acb2aaec46b146f04
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+++ b/sky/engine/core/painting/Matrix4.dart
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+// Copyright 2015 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.
+
+part of dart.sky;
+
+/// Holds a 4x4 Matrix in column-major format. Designed as a lightweight
+/// wrapper around a Float32List, used for passing a Matrix4 to C++.
+class Matrix4 {
+  final Float32List _value;
+  Float32List get storage => _value;
+
+  Matrix4(this._value);
+}