Implement spawnFunction from the new isolate api.

runtime/vm/zone.cc

Index: runtime/vm/zone.cc
===================================================================
--- runtime/vm/zone.cc	(revision 5367)
+++ runtime/vm/zone.cc	(working copy)
@@ -243,4 +243,21 @@
   }
 }
 
+
+char* Zone::PrintToString(const char* format, ...) {
+  va_list args;
+  va_start(args, format);
+  intptr_t len = OS::VSNPrint(NULL, 0, format, args);
+  va_end(args);
+
+  char* buffer = reinterpret_cast<char*>(Allocate(len + 1));
+  va_list args2;
+  va_start(args2, format);
+  OS::VSNPrint(buffer, (len + 1), format, args2);
+  va_end(args2);
+
+  return buffer;
+}
+
+
 }  // namespace dart