Use a signed 32-bit integer for representing code points.

runtime/lib/string.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 "vm/exceptions.h"
 #include "vm/native_entry.h"
 #include "vm/object.h"
 #include "vm/symbols.h"
 
 namespace dart {
 
 DEFINE_NATIVE_ENTRY(StringBase_createFromCodePoints, 1) {
   GET_NATIVE_ARGUMENT(Array, a, arguments->NativeArgAt(0));
   // TODO(srdjan): Check that parameterized type is an int.
   Zone* zone = isolate->current_zone();
   intptr_t array_len = a.Length();
 
   // Unbox the array and determine the maximum element width.
   bool is_one_byte_string = true;
   intptr_t utf16_len = array_len;
-  uint32_t* utf32_array = zone->Alloc<uint32_t>(array_len);
+  int32_t* utf32_array = zone->Alloc<int32_t>(array_len);
   Object& index_object = Object::Handle(isolate);
   for (intptr_t i = 0; i < array_len; i++) {
     index_object = a.At(i);
     if (!index_object.IsSmi()) {
       GrowableArray<const Object*> args;
       args.Add(&index_object);
       Exceptions::ThrowByType(Exceptions::kArgument, args);
     }
     intptr_t value = Smi::Cast(index_object).Value();
     if (value < 0) {
@@ skipping 117 matching lines @@
     if (!elem.IsString()) {
       GrowableArray<const Object*> args;
       args.Add(&elem);
       Exceptions::ThrowByType(Exceptions::kArgument, args);
     }
   }
   return String::ConcatAll(strings);
 }
 
 }  // namespace dart