Create isolates in a separate thread.

runtime/vm/isolate_test.cc

Index: runtime/vm/isolate_test.cc
diff --git a/runtime/vm/isolate_test.cc b/runtime/vm/isolate_test.cc
index 2bb21c798c39cc14dbf2c1a71f2ae66829663504..864bb70ebcb61318ce4a0ce453faf62624cd0665 100644
--- a/runtime/vm/isolate_test.cc
+++ b/runtime/vm/isolate_test.cc
@@ -73,7 +73,8 @@ TEST_CASE(IsolateSpawn) {
   EXPECT(!Dart_IsError(result));
   // Run until all ports to isolate are closed.
   result = Dart_RunLoop();
-  EXPECT_ERROR(result, "Null callback specified for isolate creation");
+  EXPECT_ERROR(result,
+               "Isolate spawn is not supported by this Dart implementation");
   EXPECT(Dart_ErrorHasException(result));
   Dart_Handle exception_result = Dart_ErrorGetException(result);
   EXPECT_VALID(exception_result);