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

runtime/vm/assembler_arm64.cc

Index: runtime/vm/assembler_arm64.cc
diff --git a/runtime/vm/assembler_arm64.cc b/runtime/vm/assembler_arm64.cc
index 2d800fc5c8b6835068470d53f3936e708573ae85..edaf0cb33f80322e1ab5f696e2ea7a4763700be6 100644
--- a/runtime/vm/assembler_arm64.cc
+++ b/runtime/vm/assembler_arm64.cc
@@ -985,12 +985,12 @@ void Assembler::StoreIntoObjectOffsetNoBarrier(Register object,
 
 
 void Assembler::LoadClassId(Register result, Register object, Register pp) {
-  ASSERT(RawObject::kClassIdTagPos == 16);
-  ASSERT(RawObject::kClassIdTagSize == 16);
+  ASSERT(RawObject::kClassIdTagPos == kBitsPerInt32);
+  ASSERT(RawObject::kClassIdTagSize == kBitsPerInt32);
   const intptr_t class_id_offset = Object::tags_offset() +
       RawObject::kClassIdTagPos / kBitsPerByte;
   LoadFromOffset(result, object, class_id_offset - kHeapObjectTag, pp,
-                 kUnsignedHalfword);
+                 kUnsignedWord);
 }