Change snapshot versioning code to use plain strings instead of Dart strings

runtime/vm/snapshot.h

Index: runtime/vm/snapshot.h
===================================================================
--- runtime/vm/snapshot.h	(revision 39704)
+++ runtime/vm/snapshot.h	(working copy)
@@ -223,6 +223,10 @@
     stream_.Advance(value);
   }
 
+  intptr_t PendingBytes() const {
+    return stream_.PendingBytes();
+  }
+
   RawSmi* ReadAsSmi();
   intptr_t ReadSmiValue();
 
@@ -277,6 +281,9 @@
   // Read a script snap shot.
   RawObject* ReadScriptSnapshot();
 
+  // Read version number of snapshot and verify.
+  RawApiError* VerifyVersion();
+
   // Helper functions for creating uninitialized versions
   // of various object types. These are used when reading a
   // full snapshot.
@@ -594,6 +601,9 @@
   }
   void ThrowException(Exceptions::ExceptionType type, const char* msg);
 
+  // Write a version string for the snapshot.
+  void WriteVersion();
+
  protected:
   void UnmarkAll() {
     forward_list_.UnmarkAll();