Don't test directly against Scanner::kNoSourcePos instead use >= 0.

runtime/vm/parser.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/parser.h"
 #include "vm/flags.h"
 
 #ifndef DART_PRECOMPILED
 
 #include "lib/invocation_mirror.h"
@@ skipping 7549 matching lines @@
     variable = new(Z) LocalVariable(
         assign_pos, ident, type);
     AstNode* null_expr = new(Z) LiteralNode(ident_pos, Object::null_instance());
     initialization = new(Z) StoreLocalNode(
         ident_pos, variable, null_expr);
   }
 
   ASSERT(current_block_ != NULL);
   const intptr_t previous_pos =
       current_block_->scope->PreviousReferencePos(ident);
-  if (previous_pos != Scanner::kNoSourcePos) {
+  if (previous_pos >= 0) {
     ASSERT(!script_.IsNull());
     if (previous_pos > ident_pos) {
       ReportError(ident_pos,
                   "initializer of '%s' may not refer to itself",
                   ident.ToCString());
 
     } else {
       intptr_t line_number;
       script_.GetTokenLocation(previous_pos, &line_number, NULL);
       ReportError(ident_pos,
@@ skipping 6996 matching lines @@
     const ArgumentListNode& function_args,
     const LocalVariable* temp_for_last_arg,
     bool is_super_invocation) {
   UNREACHABLE();
   return NULL;
 }
 
 }  // namespace dart
 
 #endif  // DART_PRECOMPILED