Remove usage of Dart_AllocateWithNativeFields from tonic (flutter#16588)

This API is being removed from the Dart SDK.

Author: bkonyi

lib/ui/painting.dart

@@ -1890,6 +1890,12 @@ class Path extends NativeFieldWrapperClass2 {
   /// Create a new empty [Path] object.
   @pragma('vm:entry-point')
   Path() { _constructor(); }
+
+  // Workaround for tonic, which expects classes with native fields to have a
+  // private constructor.
+  @pragma('vm:entry-point')
+  Path._() { _constructor(); }
+
   void _constructor() native 'Path_constructor';
 
   /// Creates a copy of another [Path].

third_party/tonic/dart_wrappable.cc

@@ -22,13 +22,17 @@ Dart_Handle DartWrappable::CreateDartWrapper(DartState* dart_state) {
   Dart_PersistentHandle type = dart_state->class_library().GetClass(info);
   TONIC_DCHECK(!LogIfError(type));
 
-  intptr_t native_fields[kNumberOfNativeFields];
-  native_fields[kPeerIndex] = reinterpret_cast<intptr_t>(this);
-  native_fields[kWrapperInfoIndex] = reinterpret_cast<intptr_t>(&info);
-  Dart_Handle wrapper =
-      Dart_AllocateWithNativeFields(type, kNumberOfNativeFields, native_fields);
+  Dart_Handle private_constructor_name = Dart_NewStringFromCString("_");
+  Dart_Handle wrapper = Dart_New(type, private_constructor_name, 0, nullptr);
   TONIC_DCHECK(!LogIfError(wrapper));
 
+  Dart_Handle res = Dart_SetNativeInstanceField(
+      wrapper, kPeerIndex, reinterpret_cast<intptr_t>(this));
+  TONIC_DCHECK(!LogIfError(res));
+  res = Dart_SetNativeInstanceField(wrapper, kWrapperInfoIndex,
+                                    reinterpret_cast<intptr_t>(&info));
+  TONIC_DCHECK(!LogIfError(res));
+
   this->RetainDartWrappableReference();  // Balanced in FinalizeDartWrapper.
   dart_wrapper_ = Dart_NewWeakPersistentHandle(
       wrapper, this, GetAllocationSize(), &FinalizeDartWrapper);