Declare native wrapper classes in the GPU package as base classes

lib/gpu/lib/src/context.dart

@@ -8,7 +8,7 @@ import 'dart:nativewrappers';
 /// A handle to a graphics context. Used to create and manage GPU resources.
 ///
 /// To obtain the default graphics context, use [getContext].
-class GpuContext extends NativeFieldWrapperClass1 {
+base class GpuContext extends NativeFieldWrapperClass1 {
   /// Creates a new graphics context that corresponds to the default Impeller
   /// context.
   GpuContext._createDefault() {

lib/ui/experiments/gpu.dart

@@ -147,7 +147,7 @@ class RasterPipeline {}
 /// A handle to a graphics context. Used to create and manage GPU resources.
 ///
 /// To obtain the default graphics context, use [getGpuContext].
-class GpuContext extends NativeFieldWrapperClass1 {
+base class GpuContext extends NativeFieldWrapperClass1 {
   /// Creates a new graphics context that corresponds to the default Impeller
   /// context.
   GpuContext._createDefault() {