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

dart/frog/leg/frog_leg.dart

 // 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.
 
 #library('frog_leg');
 
 #import('../../lib/uri/uri.dart');
 #import('../lang.dart', prefix: 'frog');
 #import('elements/elements.dart');
 #import('io/io.dart', prefix: 'io');
@@ skipping 36 matching lines @@
     return success;
   }
 
   spanFromNode(Node node, Script current) {
     final begin = node.getBeginToken();
     final end = node.getEndToken();
     if (begin === null || end === null) {
       cancel('cannot find tokens to produce error message for $node.');
     }
     final startOffset = begin.charOffset;
-    final endOffset = end.charOffset + end.toString().length;
+    // TODO(ahe): Compute proper end offset.
+    final endOffset = end.charOffset + 1;
     return new frog.SourceSpan(current.file, startOffset, endOffset);
   }
 
   currentScript() {
     if (currentElement === null) return null;
     CompilationUnitElement compilationUnit =
       currentElement.getCompilationUnit();
     if (compilationUnit === null) return null;
     return compilationUnit.script;
   }
@@ skipping 28 matching lines @@
     return new Script(uri, sourceFile);
   }
 
   String get legDirectory() => io.join([frog.options.libDir, '..', 'leg']);
 
   void cancel([String reason, Node node, token, instruction, element]) {
     Script script = currentScript();
     if (node !== null) {
       print(spanFromNode(node, script).toMessageString("cancel leg: $reason"));
     } else if (token !== null) {
-      String tokenString = token.toString();
       int begin = token.charOffset;
-      int end = begin + tokenString.length;
+      int end = begin + 1; // TODO(ahe): Compute proper length.
       print(script.file.getLocationMessage("cancel leg: $reason",
                                            begin, end, true));
     } else if (element !== null) {
       if (element.position() === null) {
         // Sometimes, the backend fakes up elements that have no
         // position. So we use the enclosing element instead. It is
         // not a good error location, but cancel really is "internal
         // error" or "not implemented yet", so the vicinity is good
         // enough for now.
         element = element.enclosingElement;
@@ skipping 12 matching lines @@
     }
     super.cancel(reason, node, token, instruction);
   }
 }
 
 class AbortLeg {
   final message;
   AbortLeg(this.message);
   toString() => 'Aborted due to --throw-on-error: $message';
 }