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

runtime/bin/dartutils.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 "bin/dartutils.h"
 
 #include "bin/extensions.h"
 #include "bin/directory.h"
 #include "bin/file.h"
 #include "bin/io_buffer.h"
@@ skipping 324 matching lines @@
   result = DartUtils::LoadSource(NULL,
                                  library,
                                  url,
                                  tag,
                                  url_string);
   return result;
 }
 
 
 const uint8_t* DartUtils::SniffForMagicNumber(const uint8_t* text_buffer,
+                                              intptr_t* buffer_len,
                                               bool* is_snapshot) {
   intptr_t len = sizeof(magic_number);
   for (intptr_t i = 0; i < len; i++) {
     if (text_buffer[i] != magic_number[i]) {
       *is_snapshot = false;
       return text_buffer;
     }
   }
   *is_snapshot = true;
+  ASSERT(*buffer_len > len);
+  *buffer_len -= len;
   return text_buffer + len;
 }
 
 
 void DartUtils::WriteMagicNumber(File* file) {
   // Write a magic number and version information into the snapshot file.
   bool bytes_written = file->WriteFully(magic_number, sizeof(magic_number));
   ASSERT(bytes_written);
 }
 
@@ skipping 12 matching lines @@
   }
   const char* script_path_cstr;
   Dart_StringToCString(script_path, &script_path_cstr);
   const char* error_msg = NULL;
   intptr_t len;
   const uint8_t* text_buffer = ReadFile(script_path_cstr, &len, &error_msg);
   if (text_buffer == NULL) {
     return Dart_Error(error_msg);
   }
   bool is_snapshot = false;
-  text_buffer = SniffForMagicNumber(text_buffer, &is_snapshot);
+  text_buffer = SniffForMagicNumber(text_buffer, &len, &is_snapshot);
   if (is_snapshot) {
     return Dart_LoadScriptFromSnapshot(text_buffer, len);
   } else {
     Dart_Handle source = Dart_NewStringFromUTF8(text_buffer, len);
     return Dart_LoadScript(resolved_script_uri, source, 0, 0);
   }
 }
 
 
 Dart_Handle DartUtils::LoadSource(CommandLineOptions* url_mapping,
@@ skipping 337 matching lines @@
 
 CObject* CObject::NewOSError(OSError* os_error) {
   CObject* error_message =
       new CObjectString(CObject::NewString(os_error->message()));
   CObjectArray* result = new CObjectArray(CObject::NewArray(3));
   result->SetAt(0, new CObjectInt32(CObject::NewInt32(kOSError)));
   result->SetAt(1, new CObjectInt32(CObject::NewInt32(os_error->code())));
   result->SetAt(2, error_message);
   return result;
 }