Return null from stringValue and call slowToString() instead of toString().

dart/frog/leg/ssa/tracer.dart

 // Copyright (c) 2011, 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.
 
 #library('tracer');
 
 #import('dart:io');
 #import('ssa.dart');
 
 class HTracer extends HGraphVisitor {
@@ skipping 195 matching lines @@
 
   String visitBoundsCheck(HBoundsCheck node) {
     String lengthId = temporaryId(node.length);
     String indexId = temporaryId(node.index);
     return "Bounds check: length = $lengthId, index = $indexId";
   }
 
   String visitBreak(HBreak node) {
     HBasicBlock target = currentBlock.successors[0];
     if (node.label !== null) {
-      return "Break ${node.label.stringValue}: (B${target.id})";
+      return "Break ${node.label.slowToString()}: (B${target.id})";
     }
     return "Break: (B${target.id})";
   }
 
   String visitDivide(HDivide node) => visitInvokeStatic(node);
 
   String visitEquals(HEquals node) => visitInvokeStatic(node);
 
   String visitExit(HExit node) => "exit";
 
@@ skipping 105 matching lines @@
       if (i != 0) elementsString.add(", ");
       elementsString.add(temporaryId(node.inputs[i]));
     }
     return "Literal list: [$elementsString]";
   }
 
   String visitLoad(HLoad node) => "Load: ${temporaryId(node.inputs[0])}";
 
   String visitLocal(HLocal node) {
     if (node.element !== null) {
-      return "Local: ${node.element.name.stringValue}";
+      return "Local: ${node.element.name.slowToString()}";
     } else {
       return "Local";
     }
   }
 
   String visitLogicalOperator(HLogicalOperator node) {
     return "LogicalOepration: ${temporaryId(node.left)} ${node.operation} " +
         "${temporaryId(node.right)}";
   }
 
@@ skipping 76 matching lines @@
       default: unreachable();
     }
     return "TypeGuard: ${temporaryId(node.inputs[0])} is $type";
   }
 
   String visitIs(HIs node) {
     String type = node.typeExpression.toString();
     return "TypeTest: ${temporaryId(node.expression)} is $type";
   }
 }