[fuchsia] NNBD: Kernel libraries for Flutter, Dart runners (#18714)

This updates the references to `third_party/dart/sdk` to
`third_party/dart/sdk_nnbd`

Fixes: flutter/flutter#58431

Author: iskakaushik

shell/platform/fuchsia/dart-pkg/fuchsia/lib/fuchsia.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart = 2.6
 library fuchsia;
 
 import 'dart:io';

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

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart = 2.6
 part of zircon;
 
 // ignore_for_file: native_function_body_in_non_sdk_code

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

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart = 2.6
 part of zircon;
 
 // ignore_for_file: native_function_body_in_non_sdk_code

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

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart = 2.6
 part of zircon;
 
 // ignore_for_file: native_function_body_in_non_sdk_code

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

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart = 2.6
 library zircon;
 
 import 'dart:convert' show utf8;

shell/platform/fuchsia/dart_runner/embedder/builtin.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart = 2.6
 library fuchsia_builtin;
 
 import 'dart:async';

shell/platform/fuchsia/dart_runner/kernel/BUILD.gn

@@ -20,6 +20,9 @@ compile_platform("kernel_platform_files") {
   ]
 
   args = [
+    "--enable-experiment=non-nullable",
+    "--nnbd-weak",
+
     # TODO(dartbug.com/36342): enable bytecode for core libraries when performance of bytecode
     # pipeline is on par with default pipeline and continuously tracked.
     # "--bytecode",

shell/platform/fuchsia/dart_runner/kernel/libraries.json

@@ -20,6 +20,9 @@ compile_platform("kernel_platform_files") {
   ]
 
   args = [
+    "--enable-experiment=non-nullable",
+    "--nnbd-weak",
+
     # TODO(dartbug.com/36342): enable bytecode for core libraries when performance of bytecode
     # pipeline is on par with default pipeline and continuously tracked.
     # "--bytecode",