Adds an assertion to limit snapshot intptr_t size.

runtime/vm/snapshot.h

Index: runtime/vm/snapshot.h
===================================================================
--- runtime/vm/snapshot.h	(revision 37116)
+++ runtime/vm/snapshot.h	(working copy)
@@ -181,6 +181,11 @@
     return static_cast<intptr_t>(value);
   }
 
+  intptr_t ReadRawPointerValue() {
+    int64_t value = Read<int64_t>();
+    return static_cast<intptr_t>(value);
+  }
+
   void ReadBytes(uint8_t* addr, intptr_t len) {
     stream_.ReadBytes(addr, len);
   }
@@ -399,9 +404,14 @@
 
   // Writes an intptr_t type value out.
   void WriteIntptrValue(intptr_t value) {
+    ASSERT((value >= kMinInt32) && (value <= kMaxInt32));
     Write<int64_t>(value);
   }
 
+  void WriteRawPointerValue(intptr_t value) {
+    Write<int64_t>(value);
+  }
+
   // Write an object that is serialized as an Id (singleton in object store,
   // or an object that was already serialized before).
   void WriteIndexedObject(intptr_t object_id) {