API functions returning Promises should not throw exceptions.

Source/bindings/scripts/v8_types.py

Index: Source/bindings/scripts/v8_types.py
diff --git a/Source/bindings/scripts/v8_types.py b/Source/bindings/scripts/v8_types.py
index d6acc6067ffb8b9cb9f3d29932609987a944a021..574eedd851d5d59380a1348e01b869145e47f6c6 100644
--- a/Source/bindings/scripts/v8_types.py
+++ b/Source/bindings/scripts/v8_types.py
@@ -488,7 +488,7 @@ def v8_value_to_cpp_value_array_or_sequence(native_array_element_type, v8_value,
     return expression
 
 
-def v8_value_to_local_cpp_value(idl_type, extended_attributes, v8_value, variable_name, index=None, declare_variable=True, isolate='info.GetIsolate()', used_in_private_script=False):
+def v8_value_to_local_cpp_value(idl_type, extended_attributes, v8_value, variable_name, index=None, declare_variable=True, isolate='info.GetIsolate()', used_in_private_script=False, async=False):
     """Returns an expression that converts a V8 value to a C++ value and stores it as a local value."""
 
     # FIXME: Support union type.
@@ -515,6 +515,14 @@ def v8_value_to_local_cpp_value(idl_type, extended_attributes, v8_value, variabl
     # v8::TryCatch.
     suffix = ''
 
+    suffix = ''
+
+    if async:
+        suffix += '_ASYNC'
+        args.append('info')
+        if macro == 'TONATIVE_VOID_EXCEPTIONSTATE':
+            args.insert(0, 'ScriptState::current(%s)' % isolate)
+
     if declare_variable:
         args.insert(0, this_cpp_type)
     else: