Declare native wrapper classes in Fuchsia packages as base classes (#46305)

This prohibits other implementations of the class interface that can not act as native wrappers.

See flutter/flutter#123756

Author: jason-simmons

shell/platform/fuchsia/dart-pkg/zircon/lib/src/handle.dart

@@ -8,7 +8,7 @@ part of zircon;
 // ignore_for_file: public_member_api_docs
 
 @pragma('vm:entry-point')
-class Handle extends NativeFieldWrapperClass1 {
+base class Handle extends NativeFieldWrapperClass1 {
   // No public constructor - this can only be created from native code.
   @pragma('vm:entry-point')
   Handle._();

shell/platform/fuchsia/dart-pkg/zircon/lib/src/handle_disposition.dart

@@ -8,7 +8,7 @@ part of zircon;
 // ignore_for_file: public_member_api_docs
 
 @pragma('vm:entry-point')
-class HandleDisposition extends NativeFieldWrapperClass1 {
+base class HandleDisposition extends NativeFieldWrapperClass1 {
   @pragma('vm:entry-point')
   HandleDisposition(int operation, Handle handle, int type, int rights) {
     _constructor(operation, handle, type, rights);

shell/platform/fuchsia/dart-pkg/zircon/lib/src/handle_waiter.dart

@@ -10,7 +10,7 @@ part of zircon;
 typedef AsyncWaitCallback = void Function(int status, int pending);
 
 @pragma('vm:entry-point')
-class HandleWaiter extends NativeFieldWrapperClass1 {
+base class HandleWaiter extends NativeFieldWrapperClass1 {
   // Private constructor.
   @pragma('vm:entry-point')
   HandleWaiter._();

shell/platform/fuchsia/dart-pkg/zircon/lib/src/system.dart

@@ -193,7 +193,7 @@ class MapResult extends _Result {
 }
 
 @pragma('vm:entry-point')
-class System extends NativeFieldWrapperClass1 {
+base class System extends NativeFieldWrapperClass1 {
   // No public constructor - this only has static methods.
   System._();