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

runtime/vm/unicode.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/unicode.h"
 
 #include "vm/allocation.h"
 #include "vm/globals.h"
 #include "vm/object.h"
 
@@ skipping 275 matching lines @@
   }
   if ((i < array_len) && (j == len)) {
     return false;  // output overflow
   }
   return true;  // success
 }
 
 
 bool Utf8::DecodeToUTF32(const uint8_t* utf8_array,
                          intptr_t array_len,
-                         uint32_t* dst,
+                         int32_t* dst,
                          intptr_t len) {
   intptr_t i = 0;
   intptr_t j = 0;
   intptr_t num_bytes;
   for (; (i < array_len) && (j < len); i += num_bytes, ++j) {
     int32_t ch;
     num_bytes = Utf8::Decode(&utf8_array[i], (array_len - i), &ch);
     if (ch == -1) {
       return false;  // invalid input
     }
     dst[j] = ch;
   }
   if ((i < array_len) && (j == len)) {
     return false;  // output overflow
   }
   return true;  // success
 }
 
 
 void Utf16::Encode(int32_t codepoint, uint16_t* dst) {
   ASSERT(codepoint > kMaxBmpCodepoint);
   ASSERT(dst != NULL);
   dst[0] = (Utf16::kLeadSurrogateOffset + (codepoint >> 10));
   dst[1] = (0xDC00 + (codepoint & 0x3FF));
 }
 
 }  // namespace dart