[vm/compiler] Use isolate version of AllocateContext stub.

When building method extractors in a given isolate group, the copy of
the AllocateContext stub in the isolate group should be used. Otherwise,
calls to that stub cannot be turned into PC relative calls when
appropriate.

Also retrieve the closure allocation stub from the object store directly
instead of going through GetAllocationStubForClass, now that closure
allocation is handled via a distinct stub.

Be more specific about the types of the stub objects passed to
GenerateBuildMethodExtractorStub, as they're guaranteed to be Code
objects.

TEST=Existing test cases.

Cq-Include-Trybots: luci.dart.try:vm-kernel-precomp-linux-release-x64-try,vm-kernel-precomp-linux-product-x64-try
Change-Id: I05bc5e8a133834e66af3e1003d8a6597ee11c312
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/200868
Commit-Queue: Tess Strickland <sstrickl@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>

runtime/vm/compiler/stub_code_compiler.h

@@ -53,8 +53,8 @@ class StubCodeCompiler : public AllStatic {
 #if !defined(TARGET_ARCH_IA32)
   static void GenerateBuildMethodExtractorStub(
       Assembler* assembler,
-      const Object& closure_allocation_stub,
-      const Object& context_allocation_stub);
+      const Code& closure_allocation_stub,
+      const Code& context_allocation_stub);
 #endif
 
   static void EnsureIsNewOrRemembered(Assembler* assembler,

runtime/vm/compiler/stub_code_compiler_arm.cc

@@ -223,8 +223,8 @@ void StubCodeCompiler::GenerateSharedStub(
 // SP+0: The object from which we are tearing a method off.
 void StubCodeCompiler::GenerateBuildMethodExtractorStub(
     Assembler* assembler,
-    const Object& closure_allocation_stub,
-    const Object& context_allocation_stub) {
+    const Code& closure_allocation_stub,
+    const Code& context_allocation_stub) {
   const intptr_t kReceiverOffset = target::frame_layout.param_end_from_fp + 1;
 
   __ EnterStubFrame();

runtime/vm/compiler/stub_code_compiler_arm64.cc

@@ -457,8 +457,8 @@ void StubCodeCompiler::GenerateJITCallbackTrampolines(
 // R4: The type_arguments_field_offset (or 0)
 void StubCodeCompiler::GenerateBuildMethodExtractorStub(
     Assembler* assembler,
-    const Object& closure_allocation_stub,
-    const Object& context_allocation_stub) {
+    const Code& closure_allocation_stub,
+    const Code& context_allocation_stub) {
   const intptr_t kReceiverOffset = target::frame_layout.param_end_from_fp + 1;
 
   __ EnterStubFrame();

runtime/vm/compiler/stub_code_compiler_x64.cc

@@ -400,8 +400,8 @@ void StubCodeCompiler::GenerateJITCallbackTrampolines(
 // RDX: The type_arguments_field_offset (or 0)
 void StubCodeCompiler::GenerateBuildMethodExtractorStub(
     Assembler* assembler,
-    const Object& closure_allocation_stub,
-    const Object& context_allocation_stub) {
+    const Code& closure_allocation_stub,
+    const Code& context_allocation_stub) {
   const intptr_t kReceiverOffsetInWords =
       target::frame_layout.param_end_from_fp + 1;
 

runtime/vm/stub_code.cc

@@ -306,11 +306,10 @@ CodePtr StubCode::GetBuildMethodExtractorStub(
   auto Z = thread->zone();
   auto object_store = thread->isolate_group()->object_store();
 
-  const auto& closure_class =
-      Class::ZoneHandle(Z, object_store->closure_class());
   const auto& closure_allocation_stub =
-      Code::ZoneHandle(Z, StubCode::GetAllocationStubForClass(closure_class));
-  const auto& context_allocation_stub = StubCode::AllocateContext();
+      Code::ZoneHandle(Z, object_store->allocate_closure_stub());
+  const auto& context_allocation_stub =
+      Code::ZoneHandle(Z, object_store->allocate_context_stub());
 
   compiler::ObjectPoolBuilder object_pool_builder;
   compiler::Assembler assembler(pool != nullptr ? pool : &object_pool_builder);