Eliminate some DebugStep calls in async code

runtime/vm/flow_graph_builder.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/flow_graph_builder.h"
 
 #include "lib/invocation_mirror.h"
 #include "vm/ast_printer.h"
 #include "vm/bit_vector.h"
 #include "vm/class_finalizer.h"
@@ skipping 3418 matching lines @@
   Definition* load = BuildLoadLocal(node->local());
   ReturnDefinition(load);
 }
 
 
 // <Expression> ::= StoreLocal { local: LocalVariable
 //                               value: <Expression> }
 void EffectGraphVisitor::VisitStoreLocalNode(StoreLocalNode* node) {
   // If the right hand side is an expression that does not contain
   // a safe point for the debugger to stop, add an explicit stub
-  // call.
-  if (node->value()->IsLiteralNode() ||
+  // call. Exception: don't do this when assigning to internal variables,
+  // or for generated code that has no source position.
+  if ((node->value()->IsLiteralNode() ||
       node->value()->IsLoadLocalNode() ||
-      node->value()->IsClosureNode()) {
+      node->value()->IsClosureNode()) &&
+      node->local().IsInternal() &&
+      (node->token_pos() != Scanner::kNoSourcePos)) {
     AddInstruction(new(Z) DebugStepCheckInstr(
         node->token_pos(), RawPcDescriptors::kRuntimeCall));
   }
 
   ValueGraphVisitor for_value(owner());
   node->value()->Visit(&for_value);
   Append(for_value);
   Value* store_value = for_value.value();
   if (Isolate::Current()->TypeChecksEnabled()) {
     store_value = BuildAssignableValue(node->value()->token_pos(),
@@ skipping 912 matching lines @@
   Report::MessageF(Report::kBailout,
                    Script::Handle(function.script()),
                    function.token_pos(),
                    "FlowGraphBuilder Bailout: %s %s",
                    String::Handle(function.name()).ToCString(),
                    reason);
   UNREACHABLE();
 }
 
 }  // namespace dart