The null tests that I added for Dart_NewString{8,16,32} were too restrictive.

vm/dart_api_impl.cc

 // Copyright (c) 2012, 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 "include/dart_api.h"
 
 #include "vm/bigint_operations.h"
 #include "vm/class_finalizer.h"
 #include "vm/compiler.h"
 #include "vm/dart.h"
@@ skipping 1572 matching lines @@
                          CURRENT_FUNC);
   }
   return Api::NewHandle(isolate, String::New(str));
 }
 
 
 DART_EXPORT Dart_Handle Dart_NewString8(const uint8_t* codepoints,
                                         intptr_t length) {
   Isolate* isolate = Isolate::Current();
   DARTSCOPE(isolate);
-  if (codepoints == NULL) {
+  if (codepoints == NULL && length != 0) {
     RETURN_NULL_ERROR(codepoints);
   }
   CHECK_LENGTH(length, String::kMaxElements);
   return Api::NewHandle(isolate, String::New(codepoints, length));
 }
 
 
 DART_EXPORT Dart_Handle Dart_NewString16(const uint16_t* codepoints,
                                          intptr_t length) {
   Isolate* isolate = Isolate::Current();
   DARTSCOPE(isolate);
-  if (codepoints == NULL) {
+  if (codepoints == NULL && length != 0) {
     RETURN_NULL_ERROR(codepoints);
   }
   CHECK_LENGTH(length, String::kMaxElements);
   return Api::NewHandle(isolate, String::New(codepoints, length));
 }
 
 
 DART_EXPORT Dart_Handle Dart_NewString32(const uint32_t* codepoints,
                                          intptr_t length) {
   Isolate* isolate = Isolate::Current();
   DARTSCOPE(isolate);
-  if (codepoints == NULL) {
+  if (codepoints == NULL && length != 0) {
     RETURN_NULL_ERROR(codepoints);
   }
   CHECK_LENGTH(length, String::kMaxElements);
   return Api::NewHandle(isolate, String::New(codepoints, length));
 }
 
 
 DART_EXPORT bool Dart_IsExternalString(Dart_Handle object) {
   return RawObject::IsExternalStringClassId(Api::ClassId(object));
 }
@@ skipping 2573 matching lines @@
 DART_EXPORT void Dart_InitPerfEventsSupport(Dart_FileWriterFunction function) {
   Dart::set_perf_events_writer(function);
 }
 
 
 DART_EXPORT void Dart_InitFlowGraphPrinting(Dart_FileWriterFunction function) {
   Dart::set_flow_graph_writer(function);
 }
 
 }  // namespace dart