Setup NativeAssetsApi during isolate group creation (#53329)

This initializes the `NativeAssetsApi` for native assets resolution in the isolate group creation callback.

* dart-lang/sdk#55523

## Implementation considerations

The DartIO initialization lives in its own GN target. This doesn't work for the native assets initialization due to it having to look up the `script_uri` in one of the callbacks. Since the callbacks are function pointers, we can't have a lambda that captures the script uri. So instead, the native assets initialization lives in the flutter/runtime target.

The import from dart should probably be `runtime/include/bin/native_assets_api.h` to mirror what we're doing with dart IO, rather than directly importing from `runtime/bin/native_assets.h`.

## Testing

All native asset testing is in flutter_tools, so those tests will only run once this rolls into flutter/flutter. I have done manual testing locally with a Dart branch that removes the fallback: https://dart-review.googlesource.com/c/sdk/+/370740

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: dcharkes

runtime/BUILD.gn

@@ -107,6 +107,7 @@ source_set("runtime") {
     ":test_font",
     "$dart_src/runtime:dart_api",
     "$dart_src/runtime/bin:dart_io_api",
+    "$dart_src/runtime/bin:native_assets_api",
     "//flutter/assets",
     "//flutter/common",
     "//flutter/flow",

runtime/dart_isolate.cc

@@ -25,6 +25,7 @@
 #include "fml/message_loop_task_queues.h"
 #include "fml/task_source.h"
 #include "fml/time/time_point.h"
+#include "third_party/dart/runtime/include/bin/native_assets_api.h"
 #include "third_party/dart/runtime/include/dart_api.h"
 #include "third_party/dart/runtime/include/dart_tools_api.h"
 #include "third_party/tonic/converter/dart_converter.h"
@@ -1165,6 +1166,27 @@ bool DartIsolate::DartIsolateInitializeCallback(void** child_callback_data,
   return true;
 }
 
+static void* NativeAssetsDlopenRelative(const char* path, char** error) {
+  auto* isolate_group_data =
+      static_cast<std::shared_ptr<DartIsolateGroupData>*>(
+          Dart_CurrentIsolateGroupData());
+  const std::string& script_uri = (*isolate_group_data)->GetAdvisoryScriptURI();
+  return dart::bin::NativeAssets::DlopenRelative(path, script_uri.data(),
+                                                 error);
+}
+
+static void InitDartFFIForIsolateGroup() {
+  NativeAssetsApi native_assets;
+  memset(&native_assets, 0, sizeof(native_assets));
+  native_assets.dlopen_absolute = &dart::bin::NativeAssets::DlopenAbsolute;
+  native_assets.dlopen_relative = &NativeAssetsDlopenRelative;
+  native_assets.dlopen_system = &dart::bin::NativeAssets::DlopenSystem;
+  native_assets.dlopen_executable = &dart::bin::NativeAssets::DlopenExecutable;
+  native_assets.dlopen_process = &dart::bin::NativeAssets::DlopenProcess;
+  native_assets.dlsym = &dart::bin::NativeAssets::Dlsym;
+  Dart_InitializeNativeAssetsResolver(&native_assets);
+};
+
 Dart_Isolate DartIsolate::CreateDartIsolateGroup(
     std::unique_ptr<std::shared_ptr<DartIsolateGroupData>> isolate_group_data,
     std::unique_ptr<std::shared_ptr<DartIsolate>> isolate_data,
@@ -1191,6 +1213,8 @@ Dart_Isolate DartIsolate::CreateDartIsolateGroup(
     isolate_data.release();
     // NOLINTEND(clang-analyzer-cplusplus.NewDeleteLeaks)
 
+    InitDartFFIForIsolateGroup();
+
     success = InitializeIsolate(embedder_isolate, isolate, error);
   }
   if (!success) {