Set the stack base value for thread pool threads at the start so that profile ticks for threads sho…

runtime/vm/thread.cc

Index: runtime/vm/thread.cc
diff --git a/runtime/vm/thread.cc b/runtime/vm/thread.cc
index 2e9530296e70d0bfcac66c23c26a853ecc3daa1d..e0c1d55c5ad0aad381a26d8cf60e78ad934f4704 100644
--- a/runtime/vm/thread.cc
+++ b/runtime/vm/thread.cc
@@ -331,6 +331,7 @@ void Thread::ClearStackLimit() {
 uword Thread::GetCurrentStackPointer() {
   // Since AddressSanitizer's detect_stack_use_after_return instruments the
   // C++ code to give out fake stack addresses, we call a stub in that case.
+  ASSERT(StubCode::GetStackPointer_entry() != NULL);
   uword (*func)() = reinterpret_cast<uword (*)()>(
       StubCode::GetStackPointer_entry()->EntryPoint());
   // But for performance (and to support simulators), we normally use a local.