Insert debug step check at the beginning of a function.

runtime/vm/flow_graph_builder.cc

Index: runtime/vm/flow_graph_builder.cc
diff --git a/runtime/vm/flow_graph_builder.cc b/runtime/vm/flow_graph_builder.cc
index abdd58e44ff523cd98b833ddbb8e43e63d437dec..9457956bf2f65d371e659271540ca62f499993b4 100644
--- a/runtime/vm/flow_graph_builder.cc
+++ b/runtime/vm/flow_graph_builder.cc
@@ -4198,6 +4198,11 @@ void EffectGraphVisitor::VisitSequenceNode(SequenceNode* node) {
   ASSERT((node->label() == NULL) || !is_top_level_sequence);
   NestedBlock nested_block(owner(), node);
 
+  if (FLAG_support_debugger && is_top_level_sequence) {
+    AddInstruction(new(Z) DebugStepCheckInstr(function.token_pos(),

[Cutch, 2015/07/14 19:54:36]

I wonder if you could insert this after the GraphcEntryInstr?

[rmacnak, 2015/07/14 22:28:29]

We don't seem to have the right things in scope to easily add it right where the
GraphEntry is constructed.

+                                              RawPcDescriptors::kRuntimeCall));
+  }
+
   if (num_context_variables > 0) {
     // The local scope declares variables that are captured.
     // Allocate and chain a new context (Except don't chain when at the function