Fix build break in dart2js. The snapshot length was not accounting for the magic number.

runtime/bin/dartutils.h

 // 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 BIN_DARTUTILS_H_
 #define BIN_DARTUTILS_H_
 
 #include "bin/builtin.h"
 #include "bin/utils.h"
 #include "include/dart_api.h"
@@ skipping 145 matching lines @@
   static Dart_Handle FilePathFromUri(Dart_Handle script_uri,
                                      Dart_Handle builtin_lib);
 
   static Dart_Handle ResolveUri(Dart_Handle library_url,
                                 Dart_Handle url,
                                 Dart_Handle builtin_lib);
 
   // Sniffs the specified text_buffer to see if it contains the magic number
   // representing a script snapshot. If the text_buffer is a script snapshot
   // the return value is an updated pointer to the text_buffer pointing past
-  // the magic number value.
+  // the magic number value. The 'buffer_len' parameter is also appropriately
+  // adjusted.
   static const uint8_t* SniffForMagicNumber(const uint8_t* text_buffer,
+                                            intptr_t* buffer_len,
                                             bool* is_snapshot);
 
   // Write a magic number to indicate a script snapshot file.
   static void WriteMagicNumber(File* file);
 
   // Global state that stores the original working directory..
   static const char* original_working_directory;
 
   static const char* kDartScheme;
   static const char* kDartExtensionScheme;
@@ skipping 248 matching lines @@
   void* Peer() const { return cobject_->value.as_external_byte_array.peer; }
   Dart_WeakPersistentHandleFinalizer Callback() const {
     return cobject_->value.as_external_byte_array.callback;
   }
 
  private:
   DISALLOW_COPY_AND_ASSIGN(CObjectExternalUint8Array);
 };
 
 #endif  // BIN_DARTUTILS_H_