Make return statements single steppable

runtime/vm/flow_graph_optimizer.cc

 // Copyright (c) 2013, 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_optimizer.h"
 
 #include "vm/bit_vector.h"
 #include "vm/cha.h"
 #include "vm/dart_entry.h"
 #include "vm/flow_graph_builder.h"
@@ skipping 6755 matching lines @@
     }
   }
 }
 
 
 void ConstantPropagator::VisitNativeCall(NativeCallInstr* instr) {
   SetValue(instr, non_constant_);
 }
 
 
+void ConstantPropagator::VisitDebugStepCheck(DebugStepCheckInstr* instr) {
+  // Nothing to do.
+}
+
+
 void ConstantPropagator::VisitStringFromCharCode(
     StringFromCharCodeInstr* instr) {
   const Object& o = instr->char_code()->definition()->constant_value();
   if (o.IsNull() || IsNonConstant(o)) {
     SetValue(instr, non_constant_);
   } else if (IsConstant(o)) {
     const intptr_t ch_code = Smi::Cast(o).Value();
     ASSERT(ch_code >= 0);
     if (ch_code < Symbols::kMaxOneCharCodeSymbol) {
       RawString** table = Symbols::PredefinedAddress();
@@ skipping 1498 matching lines @@
   }
 
   // Insert materializations at environment uses.
   for (intptr_t i = 0; i < exits.length(); i++) {
     CreateMaterializationAt(exits[i], alloc, alloc->cls(), *fields);
   }
 }
 
 
 }  // namespace dart