Make visualizer robust to graphs with NULL inputs.

src/compiler/generic-graph.h

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_COMPILER_GENERIC_GRAPH_H_
 #define V8_COMPILER_GENERIC_GRAPH_H_
 
 #include "src/compiler/generic-node.h"
 
 namespace v8 {
@@ skipping 16 matching lines @@
   Zone* zone_;
   NodeId next_node_id_;
 };
 
 template <class V>
 class GenericGraph : public GenericGraphBase {
  public:
   explicit GenericGraph(Zone* zone)
       : GenericGraphBase(zone), start_(NULL), end_(NULL) {}
 
-  V* start() { return start_; }
-  V* end() { return end_; }
+  V* start() const { return start_; }
+  V* end() const { return end_; }
 
   void SetStart(V* start) { start_ = start; }
   void SetEnd(V* end) { end_ = end; }
 
  private:
   V* start_;
   V* end_;
 
   DISALLOW_COPY_AND_ASSIGN(GenericGraph);
 };
 }
 }
 }  // namespace v8::internal::compiler
 
 #endif  // V8_COMPILER_GENERIC_GRAPH_H_