Refactor conversion to native.

lib/dom/scripts/systemnative.py

Index: lib/dom/scripts/systemnative.py
diff --git a/lib/dom/scripts/systemnative.py b/lib/dom/scripts/systemnative.py
index 0b1979d5743cff49656d1e67fc874e576336cc6d..ed0b0ba54aa4a33a5d6d562e894b77ba7bad4d3e 100644
--- a/lib/dom/scripts/systemnative.py
+++ b/lib/dom/scripts/systemnative.py
@@ -830,16 +830,25 @@ class NativeImplementationGenerator(systembase.BaseGenerator):
     # Emit arguments.
     start_index = 1 if needs_receiver else 0
     for i, argument in enumerate(arguments):
-      type_info = self._TypeInfo(argument.type.id)
-      self._cpp_impl_includes |= set(type_info.to_native_includes())
       argument_name = DartDomNameOfAttribute(argument)
-      type_info.emit_to_native(
-          body_emitter,
+      argument_expression, type, cls, function = self._TypeInfo(argument.type.id).to_native_info(
           argument,
           (IsOptional(argument) and not self._IsArgumentOptionalInWebCore(node, argument)) or (argument.ext_attrs.get('Optional') == 'DefaultIsNullString'),
           argument_name,
-          'Dart_GetNativeArgument(args, %i)' % (start_index + i))
-      cpp_arguments.append(type_info.argument_expression(argument_name, self._interface.id))
+          self._interface.id)
+
+      body_emitter.Emit(
+          '\n'
+          '        $TYPE $ARGUMENT_NAME = $CLS::$FUNCTION(Dart_GetNativeArgument(args, $INDEX), exception);\n'
+          '        if (exception)\n'
+          '            goto fail;\n',
+          TYPE=type,
+          ARGUMENT_NAME=argument_name,
+          CLS=cls,
+          FUNCTION=function,
+          INDEX=start_index + i)
+      self._cpp_impl_includes.add('"%s.h"' % cls)
+      cpp_arguments.append(argument_expression)
 
     body_emitter.Emit('\n')