Move utils.h and utils.cc from runtime/vm to runtime/platform

runtime/vm/bigint_store.h

-// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+// 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.
 
 #ifndef VM_BIGINT_STORE_H_
 #define VM_BIGINT_STORE_H_
 
 #include "vm/globals.h"
+#include "vm/isolate.h"
 
 namespace dart {
 
 // Use to store teporary BIGNUMs.
 class BigintStore {
  public:
   BigintStore() : bn_(NULL), bn_ctx_(NULL) {}
   ~BigintStore() {
     BN_free(bn_);
     BN_CTX_free(bn_ctx_);
   }
 
   static BigintStore* Get() {
     BigintStore* store = Isolate::Current()->bigint_store();
     if (store == NULL) {
       store = new BigintStore();
       Isolate::Current()->set_bigint_store(store);
     }
     return store;
   }
 
   BIGNUM* bn_;
   BN_CTX* bn_ctx_;
 };
 
 }  // namespace dart
 
 #endif  // VM_BIGINT_STORE_H_