Eliminate some DebugStep calls in async code

runtime/vm/scopes.h

 // 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.
 
 #ifndef VM_SCOPES_H_
 #define VM_SCOPES_H_
 
 #include "platform/assert.h"
 #include "platform/globals.h"
 #include "vm/allocation.h"
@@ skipping 61 matching lines @@
   void set_invisible(bool value) {
     is_invisible_ = value;
   }
   bool is_invisible() const { return is_invisible_; }
 
   bool is_captured_parameter() const { return is_captured_parameter_; }
   void set_is_captured_parameter(bool value) {
     is_captured_parameter_ = value;
   }
 
+  // By convention, internal variables start with a colon.
+  bool IsInternal() const {
+    return name_.CharAt(0) == ':';
+  }
+
   bool IsConst() const {
     return const_value_ != NULL;
   }
 
   void SetConstValue(const Instance& value) {
     ASSERT(value.IsZoneHandle() || value.IsReadOnlyHandle());
     const_value_ = &value;
   }
 
   const Instance* ConstValue() const {
@@ skipping 282 matching lines @@
   // List of names referenced in this scope and its children that
   // are not resolved to local variables.
   GrowableArray<NameReference*> referenced_;
 
   DISALLOW_COPY_AND_ASSIGN(LocalScope);
 };
 
 }  // namespace dart
 
 #endif  // VM_SCOPES_H_