Fix --break_at_isolate_spawn. Starting in r44655 implicit static closure function are not themselve…

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index 0bb527c3d434f54ec4fca5ac467edb348687c0b8..2bce1966c95207667ee72b24a968999e2080a0e1 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -1191,16 +1191,19 @@ static bool RunIsolate(uword parameter) {
     ASSERT(result.IsFunction());
     Function& func = Function::Handle(isolate);
     func ^= result.raw();
-    func = func.ImplicitClosureFunction();
 
     // TODO(turnidge): Currently we need a way to force a one-time
     // breakpoint for all spawned isolates to support isolate
     // debugging.  Remove this once the vmservice becomes the standard
-    // way to debug.
+    // way to debug. Set the breakpoint on the static function instead
+    // of its implicit closure function because that latter is merely
+    // a dispatcher that is marked as undebuggable.
     if (FLAG_break_at_isolate_spawn) {
       isolate->debugger()->OneTimeBreakAtEntry(func);
     }
 
+    func = func.ImplicitClosureFunction();
+
     const Array& capabilities = Array::Handle(Array::New(2));
     Capability& capability = Capability::Handle();
     capability = Capability::New(isolate->pause_capability());