Clean VM thread shutdown:

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 3ccb68b0504afae84d5d5113035bd7d482f0982d..6880eab00e7a2a101f359e88acd18d045dc29155 100644
--- a/runtime/vm/dart_api_impl.cc
+++ b/runtime/vm/dart_api_impl.cc
@@ -1263,7 +1263,7 @@ DART_EXPORT const char* Dart_VersionString() {
   return Version::String();
 }
 
-DART_EXPORT bool Dart_Initialize(
+DART_EXPORT char* Dart_Initialize(
     const uint8_t* vm_isolate_snapshot,
     Dart_IsolateCreateCallback create,
     Dart_IsolateInterruptCallback interrupt,
@@ -1279,21 +1279,19 @@ DART_EXPORT bool Dart_Initialize(
                                        file_open, file_read, file_write,
                                        file_close, entropy_source);
   if (err_msg != NULL) {
-    OS::PrintErr("Dart_Initialize: %s\n", err_msg);
-    return false;
+    return strdup(err_msg);
   }
-  return true;
+  return NULL;
 }
 
 
-DART_EXPORT bool Dart_Cleanup() {
+DART_EXPORT char* Dart_Cleanup() {
   CHECK_NO_ISOLATE(Isolate::Current());
   const char* err_msg = Dart::Cleanup();
   if (err_msg != NULL) {
-    OS::PrintErr("Dart_Cleanup: %s\n", err_msg);
-    return false;
+    return strdup(err_msg);
   }
-  return true;
+  return NULL;
 }
 
 
@@ -1364,6 +1362,10 @@ DART_EXPORT Dart_Isolate Dart_CreateIsolate(const char* script_uri,
   }
   Isolate* isolate = Dart::CreateIsolate(isolate_name, *flags);
   free(isolate_name);
+  if (isolate == NULL) {
+    *error = strdup("Isolate creation failed");
+    return reinterpret_cast<Dart_Isolate>(NULL);
+  }
   {
     StackZone zone(isolate);
     HANDLESCOPE(isolate);