[vm] Mark closure objects as shallowly immutable.

TEST=ci

Issue: #55136
Change-Id: Icb39fc386439da449845eac4d0fd0df9d776aec7
Cq-Include-Trybots: luci.dart.try:vm-linux-debug-x64-try,vm-aot-linux-debug-x64-try,vm-aot-linux-debug-simarm_x64-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/435126
Commit-Queue: Alexander Markov <[email protected]>
Auto-Submit: Tess Strickland <[email protected]>
Reviewed-by: Alexander Markov <[email protected]>

Author: sstrickl

runtime/vm/class_id.h

@@ -595,8 +595,8 @@ inline bool IsDeeplyImmutableCid(intptr_t predefined_cid) {
 
 inline bool IsShallowlyImmutableCid(intptr_t predefined_cid) {
   ASSERT(predefined_cid < kNumPredefinedCids);
-  // TODO(https://dartbug.com/55136): Mark kClosureCid as shallowly imutable.
-  return IsUnmodifiableTypedDataViewClassId(predefined_cid);
+  return predefined_cid == kClosureCid ||
+         IsUnmodifiableTypedDataViewClassId(predefined_cid);
 }
 
 // See documentation on ImmutableBit in raw_object.h

runtime/vm/object_graph_copy.cc

@@ -162,17 +162,15 @@ static bool CanShareObject(ObjectPtr obj, uword tags) {
           ->IsImmutable();
     }
 
+    if (cid == kClosureCid) {
+      // We can share a closure iff it doesn't close over any state.
+      return Closure::RawCast(obj)->untag()->context() == Object::null();
+    }
+
     // All other objects that have immutability bit set are deeply immutable.
     return true;
   }
 
-  // TODO(https://dartbug.com/55136): Mark Closures as shallowly imutable.
-  // And move this into the if above.
-  if (cid == kClosureCid) {
-    // We can share a closure iff it doesn't close over any state.
-    return Closure::RawCast(obj)->untag()->context() == Object::null();
-  }
-
   return false;
 }
 
@@ -238,7 +236,7 @@ void SetNewSpaceTaggingWord(ObjectPtr to, classid_t cid, uint32_t size) {
   tags = UntaggedObject::CanonicalBit::update(false, tags);
   tags = UntaggedObject::NewOrEvacuationCandidateBit::update(true, tags);
   tags = UntaggedObject::ImmutableBit::update(
-      IsUnmodifiableTypedDataViewClassId(cid), tags);
+      Object::ShouldHaveImmutabilityBitSet(cid), tags);
 #if defined(HASH_IN_OBJECT_HEADER)
   tags = UntaggedObject::HashTag::update(0, tags);
 #endif