[vm] Remove dead read-only heap code.

This was part of the VM isolate.

TEST=ci
Change-Id: I831e984025c5d4d17607e8a36222bdef70bf66b8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/508240
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>

runtime/vm/heap/heap.cc

@@ -54,7 +54,6 @@ Heap::Heap(IsolateGroup* isolate_group,
     : isolate_group_(isolate_group),
       new_space_(this, max_new_gen_semi_words),
       old_space_(this, max_old_gen_words),
-      read_only_(false),
       assume_scavenge_will_fail_(false),
       gc_on_nth_allocation_(kNoForcedGarbageCollection) {
   UpdateGlobalMaxUsed();
@@ -683,12 +682,6 @@ void Heap::UpdateGlobalMaxUsed() {
       (UsedInWords(Heap::kOld) * kWordSize));
 }
 
-void Heap::WriteProtect(bool read_only) {
-  read_only_ = read_only;
-  new_space_.WriteProtect(read_only);
-  old_space_.WriteProtect(read_only);
-}
-
 void Heap::Init(IsolateGroup* isolate_group,
                 intptr_t max_new_gen_words,
                 intptr_t max_old_gen_words) {

runtime/vm/heap/heap.h

@@ -73,7 +73,6 @@ class Heap {
   PageSpace* old_space() { return &old_space_; }
 
   uword Allocate(Thread* thread, intptr_t size, Space space) {
-    ASSERT(!read_only_);
     switch (space) {
       case kNew:
         // Do not attempt to allocate very large objects in new space.
@@ -129,7 +128,6 @@ class Heap {
 
   // Protect access to the heap. Note: Code pages are made
   // executable/non-executable when 'read_only' is true/false, respectively.
-  void WriteProtect(bool read_only);
   void WriteProtectCode(bool read_only) {
     old_space_.WriteProtectCode(read_only);
   }
@@ -373,9 +371,6 @@ class Heap {
 
   RelaxedAtomic<Dart_PerformanceMode> mode_ = {Dart_PerformanceMode_Default};
 
-  // This heap is in read-only mode: No allocation is allowed.
-  bool read_only_;
-
   bool assume_scavenge_will_fail_;
 
   static constexpr intptr_t kNoForcedGarbageCollection = -1;

runtime/vm/heap/heap_test.cc

@@ -190,21 +190,6 @@ TEST_CASE(ClassHeapStats) {
 }
 #endif  // !PRODUCT
 
-ISOLATE_UNIT_TEST_CASE(IterateReadOnly) {
-  const String& obj = String::Handle(String::New("x", Heap::kOld));
-
-  // It is not safe to make the heap read-only if marking or sweeping is in
-  // progress.
-  GCTestHelper::WaitForGCTasks();
-
-  Heap* heap = IsolateGroup::Current()->heap();
-  EXPECT(heap->Contains(UntaggedObject::ToAddr(obj.ptr())));
-  heap->WriteProtect(true);
-  EXPECT(heap->Contains(UntaggedObject::ToAddr(obj.ptr())));
-  heap->WriteProtect(false);
-  EXPECT(heap->Contains(UntaggedObject::ToAddr(obj.ptr())));
-}
-
 ISOLATE_UNIT_TEST_CASE(CollectAllGarbage_DeadOldToNew) {
   Heap* heap = IsolateGroup::Current()->heap();