VM: Remove unnecesary load of isolate in some stubs.

runtime/vm/stub_code_arm64.cc

Index: runtime/vm/stub_code_arm64.cc
diff --git a/runtime/vm/stub_code_arm64.cc b/runtime/vm/stub_code_arm64.cc
index 1e3c78c7e08a8939c3e4db778b8b2c993052b78c..7211745eb147e6b4492b478c7dd44f86ee561fbc 100644
--- a/runtime/vm/stub_code_arm64.cc
+++ b/runtime/vm/stub_code_arm64.cc
@@ -692,8 +692,7 @@ void StubCode::GenerateAllocateArrayStub(Assembler* assembler) {
   NOT_IN_PRODUCT(__ MaybeTraceAllocation(kArrayCid, R4, &slow_case));
 
   Heap::Space space = Heap::kNew;
-  __ LoadIsolate(R8);
-  __ ldr(R8, Address(R8, Isolate::heap_offset()));
+  __ ldr(R8, Address(THR, Thread::heap_offset()));
 
   // Calculate and align allocation size.
   // Load new object start and calculate next object start.
@@ -960,8 +959,7 @@ void StubCode::GenerateAllocateContextStub(Assembler* assembler) {
     // R2: object size.
     const intptr_t cid = kContextCid;
     Heap::Space space = Heap::kNew;
-    __ LoadIsolate(R5);
-    __ ldr(R5, Address(R5, Isolate::heap_offset()));
+    __ ldr(R5, Address(THR, Thread::heap_offset()));
     __ ldr(R0, Address(R5, Heap::TopOffset(space)));
     __ add(R3, R2, Operand(R0));
     // Check if the allocation fits into the remaining space.