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

runtime/platform/globals.h

Index: runtime/platform/globals.h
diff --git a/runtime/platform/globals.h b/runtime/platform/globals.h
index 679dad14d3ac5f981597a8daf1feb5cd4c5528d1..0b2046ec4540c6958f04a4d3de436bb5d42cdfd9 100644
--- a/runtime/platform/globals.h
+++ b/runtime/platform/globals.h
@@ -341,8 +341,14 @@ const int64_t kSignBitDouble = DART_INT64_C(0x8000000000000000);
 typedef intptr_t word;
 typedef uintptr_t uword;
 
-// Size of a class id. We may make this 32-bit on 64-bit architectures.
+// Size of a class id.
+#if defined(ARCH_IS_32_BIT)
 typedef uint16_t classid_t;
+#elif defined(ARCH_IS_64_BIT)
+typedef uint32_t classid_t;
+#else
+#error Unexpected architecture word size
+#endif
 
 // Byte sizes.
 const int kWordSize = sizeof(word);