Allow stepping when paused at isolate start.

runtime/vm/debugger.cc

Index: runtime/vm/debugger.cc
diff --git a/runtime/vm/debugger.cc b/runtime/vm/debugger.cc
index c7d549c831965e25d1a4d33d9e3c3fa5f2efe638..5573f4bd9e0475cf5c09f63077ed897f74ebd8a6 100644
--- a/runtime/vm/debugger.cc
+++ b/runtime/vm/debugger.cc
@@ -1270,6 +1270,7 @@ void Debugger::SetStepOut() {
   resume_action_ = kStepOut;
 }
 
+
 RawFunction* Debugger::ResolveFunction(const Library& library,
                                        const String& class_name,
                                        const String& function_name) {
@@ -2322,6 +2323,13 @@ void Debugger::Pause(DebuggerEvent* event) {
 }
 
 
+void Debugger::EnterSingleStepMode() {
+  stepping_fp_ = 0;
+  DeoptimizeWorld();
+  isolate_->set_single_step(true);
+}
+
+
 void Debugger::HandleSteppingRequest(DebuggerStackTrace* stack_trace) {
   stepping_fp_ = 0;
   if (resume_action_ == kSingleStep) {