[kernel] Don't assume to have source

runtime/vm/object.cc

 // 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.
 
 #include "vm/object.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/assembler.h"
 #include "vm/become.h"
@@ skipping 9206 matching lines @@
     }
     *line = min + 1;  // Line numbers start at 1.
     smi ^= line_starts_array.At(min);
     if (column != NULL) {
       *column = offset - smi.Value() + 1;
     }
     if (token_len != NULL) {
       // We don't explicitly save this data: Load the source
       // and find it from there.
       const String& source = String::Handle(zone, Source());
-      ASSERT(source.Length() >= offset);
       *token_len = 1;
-      if (Scanner::IsIdentStartChar(source.CharAt(offset))) {
+      if (offset < source.Length() &&
+          Scanner::IsIdentStartChar(source.CharAt(offset))) {
         for (intptr_t i = offset + 1;
              i < source.Length() && Scanner::IsIdentChar(source.CharAt(i));
              ++i) {
           ++*token_len;
         }
       }
     }
     return;
   }
 
@@ skipping 13708 matching lines @@
   return UserTag::null();
 }
 
 
 const char* UserTag::ToCString() const {
   const String& tag_label = String::Handle(label());
   return tag_label.ToCString();
 }
 
 }  // namespace dart