Allow allocation of four byte strings through the Dart API.

runtime/vm/dart_api_impl.cc

Index: runtime/vm/dart_api_impl.cc
diff --git a/runtime/vm/dart_api_impl.cc b/runtime/vm/dart_api_impl.cc
index 26b02752a12b2979518b4a0f220a3773903f0494..007a3efc77ad2700a0b83521ec6e8ce19841a0fa 100644
--- a/runtime/vm/dart_api_impl.cc
+++ b/runtime/vm/dart_api_impl.cc
@@ -750,7 +750,6 @@ DART_EXPORT Dart_Handle Dart_NewString32(const uint32_t* codepoints,
                                          intptr_t length) {
   Zone zone;  // Setup a VM zone as we are creating some handles.
   HandleScope scope;  // Setup a VM handle scope.
-  UNIMPLEMENTED();
   const String& obj = String::Handle(String::New(codepoints, length));
   return Api::NewLocalHandle(obj);
 }