Add some support for the code-point code-unit distinction.

runtime/vm/dart_api_message.cc

Index: runtime/vm/dart_api_message.cc
diff --git a/runtime/vm/dart_api_message.cc b/runtime/vm/dart_api_message.cc
index 64eea311b36bbefba79d1888f3a34d9b83c4fc4d..0cacfa5d9e7ac10f4cf641cc17dc0bc5c909439c 100644
--- a/runtime/vm/dart_api_message.cc
+++ b/runtime/vm/dart_api_message.cc
@@ -776,7 +776,7 @@ bool ApiMessageWriter::WriteCObjectInlined(Dart_CObject* object,
       }
 
       Utf8::Type type;
-      intptr_t len = Utf8::CodePointCount(utf8_str, utf8_len, &type);
+      intptr_t len = Utf8::CodeUnitCount(utf8_str, utf8_len, &type);
 
       // Write out the serialization header value for this object.
       WriteInlinedHeader(object);