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

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index 612342ab7b4d53962340f35d332b726d65c01f52..6a4068490c7f2f6486c38e7a9b258937c19f1719 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -9964,7 +9964,7 @@ intptr_t String::Hash(const uint16_t* characters, intptr_t len) {
 }
 
 
-intptr_t String::Hash(const uint32_t* characters, intptr_t len) {
+intptr_t String::Hash(const int32_t* characters, intptr_t len) {
   return HashImpl(characters, len);
 }
 
@@ -10080,7 +10080,7 @@ bool String::Equals(const uint16_t* utf16_array, intptr_t len) const {
 }
 
 
-bool String::Equals(const uint32_t* utf32_array, intptr_t len) const {
+bool String::Equals(const int32_t* utf32_array, intptr_t len) const {
   CodePointIterator it(*this);
   intptr_t i = 0;
   while (it.Next()) {
@@ -10186,7 +10186,7 @@ RawString* String::New(const uint16_t* utf16_array,
 }
 
 
-RawString* String::New(const uint32_t* utf32_array,
+RawString* String::New(const int32_t* utf32_array,
                        intptr_t array_len,
                        Heap::Space space) {
   bool is_one_byte_string = true;
@@ -10740,7 +10740,7 @@ RawOneByteString* OneByteString::New(const uint16_t* characters,
 }
 
 
-RawOneByteString* OneByteString::New(const uint32_t* characters,
+RawOneByteString* OneByteString::New(const int32_t* characters,
                                      intptr_t len,
                                      Heap::Space space) {
   const String& result = String::Handle(OneByteString::New(len, space));
@@ -10875,7 +10875,7 @@ RawTwoByteString* TwoByteString::New(const uint16_t* utf16_array,
 
 
 RawTwoByteString* TwoByteString::New(intptr_t utf16_len,
-                                     const uint32_t* utf32_array,
+                                     const int32_t* utf32_array,
                                      intptr_t array_len,
                                      Heap::Space space) {
   ASSERT((array_len > 0) && (utf16_len >= array_len));