Move image loading out of C++ into Dart

sky/engine/tonic/mojo_converter.h

 // 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.
 
 #ifndef SKY_ENGINE_TONIC_MOJO_CONVERTER_H_
 #define SKY_ENGINE_TONIC_MOJO_CONVERTER_H_
 
 #include "mojo/public/cpp/system/handle.h"
 #include "sky/engine/tonic/dart_converter.h"
 
 namespace blink {
 
 template <typename HandleType>
 struct DartConverter<mojo::ScopedHandleBase<HandleType>> {
   static mojo::ScopedHandleBase<HandleType> FromDart(Dart_Handle handle) {
     uint64_t mojo_handle64 = 0;
     Dart_Handle result = Dart_IntegerToUint64(handle, &mojo_handle64);
     if (Dart_IsError(result) || !mojo_handle64)
       return mojo::ScopedHandleBase<HandleType>();
 
     HandleType mojo_handle(static_cast<MojoHandle>(mojo_handle64));
     return mojo::MakeScopedHandle(mojo_handle);
   }
 
   static Dart_Handle ToDart(mojo::ScopedHandleBase<HandleType> mojo_handle) {
     return Dart_NewInteger(static_cast<int64_t>(mojo_handle.release().value()));
   }
+
+  static mojo::ScopedHandleBase<HandleType> FromArgumentsWithNullCheck(
+      Dart_NativeArguments args,
+      int index,
+      Dart_Handle& exception,
+      bool auto_scope = true) {
+    int64_t mojo_handle64 = 0;
+    Dart_Handle result = Dart_GetNativeIntegerArgument(args, index, &mojo_handle64);
+    if (Dart_IsError(result) || !mojo_handle64)
+      return mojo::ScopedHandleBase<HandleType>();
+
+    HandleType mojo_handle(static_cast<MojoHandle>(mojo_handle64));
+    return mojo::MakeScopedHandle(mojo_handle);
+  }
 };
 
 }  // namespace blink
 
 #endif  // SKY_ENGINE_TONIC_MOJO_CONVERTER_H_