Expand the class id to 32 bits and size field to 16 bits on 64-bit platforms.

runtime/vm/snapshot.h

Index: runtime/vm/snapshot.h
diff --git a/runtime/vm/snapshot.h b/runtime/vm/snapshot.h
index 13c2ecdce0fe0ddd66cec19e13201ac3d398ee77..d6241ae60eb5e234c00760f75283375b9f7fe131 100644
--- a/runtime/vm/snapshot.h
+++ b/runtime/vm/snapshot.h
@@ -195,6 +195,12 @@ class BaseReader {
     return static_cast<intptr_t>(value);
   }
 
+  classid_t ReadClassIDValue() {
+    uint32_t value = Read<uint32_t>();
+    return static_cast<classid_t>(value);
+  }
+  COMPILE_ASSERT(sizeof(uint32_t) >= sizeof(classid_t));
+
   void ReadBytes(uint8_t* addr, intptr_t len) {
     stream_.ReadBytes(addr, len);
   }
@@ -511,6 +517,11 @@ class BaseWriter : public StackResource {
     Write<int64_t>(value);
   }
 
+  void WriteClassIDValue(classid_t value) {
+    Write<uint32_t>(value);
+  }
+  COMPILE_ASSERT(sizeof(uint32_t) >= sizeof(classid_t));
+
   // 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) {