Checks snapshot validity using a hash of source file instead of a version string.

runtime/vm/version_in.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/version.h"
 
 #include "vm/cpu.h"
 #include "vm/os.h"
 
 namespace dart {
@@ skipping 10 matching lines @@
     intptr_t len = OS::SNPrint(NULL, 0, format, str_, os, arch);
     char* buffer = reinterpret_cast<char*>(malloc(len + 1));
     OS::SNPrint(buffer, (len + 1), format, str_, os, arch);
     formatted_version = buffer;
   }
   return formatted_version;
 }
 
 
 const char* Version::SnapshotString() {
-  return str_;
+  return snapshot_hash_;
 }
 
-
+const char* Version::snapshot_hash_ = "{{SNAPSHOT_HASH}}";
 const char* Version::str_ = "{{VERSION_STR}} ({{BUILD_TIME}})";
 
 }  // namespace dart