Avoid UTF-8 decoding in num.toString.

runtime/lib/object.cc

 // Copyright (c) 2011, 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 "vm/bootstrap_natives.h"
 
 #include "lib/invocation_mirror.h"
 #include "vm/code_patcher.h"
 #include "vm/exceptions.h"
 #include "vm/heap.h"
@@ skipping 28 matching lines @@
   const Instance& instance = Instance::CheckedHandle(arguments->NativeArgAt(0));
   GET_NON_NULL_NATIVE_ARGUMENT(Smi, hash, arguments->NativeArgAt(1));
   Heap* heap = isolate->heap();
   heap->SetHash(instance.raw(), hash.Value());
   return Object::null();
 }
 
 
 DEFINE_NATIVE_ENTRY(Object_toString, 1) {
   const Instance& instance = Instance::CheckedHandle(arguments->NativeArgAt(0));
+  ASSERT(!instance.IsString());  // See issue 20583.
   const char* c_str = instance.ToCString();
   return String::New(c_str);
 }
 
 
 DEFINE_NATIVE_ENTRY(Object_noSuchMethod, 6) {
   const Instance& instance = Instance::CheckedHandle(arguments->NativeArgAt(0));
   GET_NON_NULL_NATIVE_ARGUMENT(Bool, is_method, arguments->NativeArgAt(1));
   GET_NON_NULL_NATIVE_ARGUMENT(String, member_name, arguments->NativeArgAt(2));
   GET_NON_NULL_NATIVE_ARGUMENT(Smi, invocation_type, arguments->NativeArgAt(3));
@@ skipping 238 matching lines @@
   // Currently all errors are Dart instances, e.g. I/O errors
   // created by deferred loading code. LanguageErrors from
   // failed loading or finalization attempts are propagated and result
   // in the isolate's death.
   const Instance& error = Instance::Handle(prefix.LoadError());
   return error.raw();
 }
 
 
 }  // namespace dart