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

runtime/vm/raw_object.h

Index: runtime/vm/raw_object.h
diff --git a/runtime/vm/raw_object.h b/runtime/vm/raw_object.h
index 0c69b39f82d7178db69aacb59d41b6b695bf1fb0..0dc6aa381492b7f1b9cd68139ed6b843dd2dc03c 100644
--- a/runtime/vm/raw_object.h
+++ b/runtime/vm/raw_object.h
@@ -238,14 +238,27 @@ class RawObject {
     kCanonicalBit = 2,
     kFromSnapshotBit = 3,
     kRememberedBit = 4,
-    kReservedTagPos = 5,  // kReservedBit{10K,100K,1M,10M}
+#if defined(ARCH_IS_32_BIT)
+    kReservedTagPos = 5,  // kReservedBit{100K,1M,10M}
     kReservedTagSize = 3,
     kSizeTagPos = kReservedTagPos + kReservedTagSize,  // = 8
     kSizeTagSize = 8,
     kClassIdTagPos = kSizeTagPos + kSizeTagSize,  // = 16
     kClassIdTagSize = 16,
+#elif defined(ARCH_IS_64_BIT)
+    kReservedTagPos = 5,  // kReservedBit{100K,1M,10M}
+    kReservedTagSize = 11,
+    kSizeTagPos = kReservedTagPos + kReservedTagSize,  // = 16
+    kSizeTagSize = 16,
+    kClassIdTagPos = kSizeTagPos + kSizeTagSize,  // = 32
+    kClassIdTagSize = 32,
+#else
+#error Unexpected architecture word size
+#endif
   };
 
+  COMPILE_ASSERT(kClassIdTagSize == (sizeof(classid_t) * kBitsPerByte));
+
   // Encodes the object size in the tag in units of object alignment.
   class SizeTag {
    public: