Use the return value of vm native methods to set the return value,

lib/error.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "lib/error.h"
 
 #include "vm/bootstrap_natives.h"
 #include "vm/exceptions.h"
 #include "vm/object_store.h"
 #include "vm/runtime_entry.h"
@@ skipping 28 matching lines @@
   intptr_t from_line, from_column;
   script.GetTokenLocation(assertion_start, &from_line, &from_column);
   intptr_t to_line, to_column;
   script.GetTokenLocation(assertion_end, &to_line, &to_column);
   Exceptions::SetField(assertion_error, cls, "failedAssertion", String::Handle(
       script.GetSnippet(from_line, from_column, to_line, to_column)));
 
   // Throw AssertionError instance.
   Exceptions::Throw(assertion_error);
   UNREACHABLE();
+  return Object::null();
 }
 
 
 // Allocate and throw a new TypeError.
 // Arg0: index of the token of the failed type check.
 // Arg1: src value.
 // Arg2: dst type name.
 // Arg3: dst name.
 // Arg4: malformed type error message.
 // Return value: none, throws an exception.
 DEFINE_NATIVE_ENTRY(TypeError_throwNew, 5) {
   // No need to type check the arguments. This function can only be called
   // internally from the VM.
   intptr_t location = Smi::CheckedHandle(arguments->At(0)).Value();
   const Instance& src_value = Instance::CheckedHandle(arguments->At(1));
   const String& dst_type_name = String::CheckedHandle(arguments->At(2));
   const String& dst_name = String::CheckedHandle(arguments->At(3));
   const String& malformed_error = String::CheckedHandle(arguments->At(4));
   const String& src_type_name =
       String::Handle(Type::Handle(src_value.GetType()).UserVisibleName());
   Exceptions::CreateAndThrowTypeError(location, src_type_name,
                                       dst_type_name, dst_name, malformed_error);
+  UNREACHABLE();
+  return Object::null();
 }
 
 
 // Allocate and throw a new FallThroughError.
 // Arg0: index of the case clause token into which we fall through.
 // Return value: none, throws an exception.
 DEFINE_NATIVE_ENTRY(FallThroughError_throwNew, 1) {
   GET_NATIVE_ARGUMENT(Smi, smi_pos, arguments->At(0));
   intptr_t fallthrough_pos = smi_pos.Value();
 
@@ skipping 10 matching lines @@
   Exceptions::SetField(fallthrough_error, cls, "url",
                        String::Handle(script.url()));
   intptr_t line, column;
   script.GetTokenLocation(fallthrough_pos, &line, &column);
   Exceptions::SetField(fallthrough_error, cls, "line",
                        Smi::Handle(Smi::New(line)));
 
   // Throw FallThroughError instance.
   Exceptions::Throw(fallthrough_error);
   UNREACHABLE();
+  return Object::null();
 }
 
 
 // Allocate and throw StaticResolutionException.
 // Arg0: index of the static call that was not resolved at compile time.
 // Return value: none, throws an exception.
 DEFINE_NATIVE_ENTRY(StaticResolutionException_throwNew, 1) {
   GET_NATIVE_ARGUMENT(Smi, smi_pos, arguments->At(0));
   intptr_t call_pos = smi_pos.Value();
   // Allocate a new instance of type StaticResolutionException.
   const Instance& resolution_exception =
       Instance::Handle(Exceptions::NewInstance("StaticResolutionException"));
   ASSERT(!resolution_exception.IsNull());
 
   // Initialize 'url', 'line', and 'column' fields.
   DartFrameIterator iterator;
   iterator.NextFrame();  // Skip native call.
   const Script& script = Script::Handle(Exceptions::GetCallerScript(&iterator));
   const Class& cls = Class::Handle(resolution_exception.clazz());
   Exceptions::SetLocationFields(resolution_exception, cls, script, call_pos);
 
   intptr_t line, column;
   script.GetTokenLocation(call_pos, &line, &column);
   Exceptions::SetField(resolution_exception, cls, "failedResolutionLine",
                        String::Handle(script.GetLine(line)));
 
   Exceptions::Throw(resolution_exception);
   UNREACHABLE();
+  return Object::null();
 }
 
 }  // namespace dart