[vm] Remove dead Isolate::boxed_field_list_.

TEST=ci
Change-Id: I072ab19a8195c5b1cf1b32a6541a31a25bc8a1fc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/439343
Reviewed-by: Alexander Aprelev <[email protected]>
Commit-Queue: Ryan Macnak <[email protected]>

Author: rmacnak-google

runtime/vm/isolate.cc

@@ -355,8 +355,6 @@ IsolateGroup::IsolateGroup(std::shared_ptr<IsolateGroupSource> source,
       kernel_data_class_cache_mutex_(),
       kernel_constants_mutex_(),
       shared_field_initializer_rwlock_(),
-      field_list_mutex_(),
-      boxed_field_list_(GrowableObjectArray::null()),
       program_lock_(new SafepointRwLock(SafepointLevel::kGCAndDeopt)),
       active_mutators_monitor_(new Monitor()),
       max_active_mutators_(Scavenger::MaxMutatorThreadCount()),
@@ -2968,13 +2966,6 @@ void IsolateGroup::VisitSharedPointers(ObjectPointerVisitor* visitor,
     case kSharedFieldTable:
       shared_field_table()->VisitObjectPointers(visitor);
       break;
-    case kBoxedFieldList:
-      // Visit the boxed_field_list_.
-      // 'boxed_field_list_' access via mutator and background compilation
-      // threads is guarded with a monitor. This means that we can visit it only
-      // when at safepoint or the field_list_mutex_ lock has been taken.
-      visitor->VisitPointer(reinterpret_cast<ObjectPtr*>(&boxed_field_list_));
-      break;
     case kBackgroundCompiler:
       NOT_IN_PRECOMPILED(background_compiler()->VisitPointers(visitor));
       break;

runtime/vm/isolate.h

@@ -271,7 +271,6 @@ enum RootSlice : intptr_t {
   kSentinelFieldTable,
   kSharedInitialFieldTable,
   kSharedFieldTable,
-  kBoxedFieldList,
   kBackgroundCompiler,
   kDebugger,
   kReloadContext,
@@ -300,8 +299,6 @@ inline const char* RootSliceToCString(intptr_t slice) {
       return "shared initial field table";
     case kSharedFieldTable:
       return "shared field table";
-    case kBoxedFieldList:
-      return "boxed field list";
     case kBackgroundCompiler:
       return "background compiler";
     case kDebugger:
@@ -984,11 +981,6 @@ class IsolateGroup : public IntrusiveDListEntry<IsolateGroup> {
   // Ensure exclusive execution of shared field initializers.
   SafepointRwLock shared_field_initializer_rwlock_;
 
-  // Protect access to boxed_field_list_.
-  Mutex field_list_mutex_;
-  // List of fields that became boxed and that trigger deoptimization.
-  GrowableObjectArrayPtr boxed_field_list_;
-
   // Ensures synchronized access to classes functions, fields and other
   // program structure elements to accommodate concurrent modification done
   // by multiple isolates and background compiler.