VM: Fix allocation size stats for contexts on ia32 and x64.

runtime/vm/stub_code_ia32.cc

Index: runtime/vm/stub_code_ia32.cc
diff --git a/runtime/vm/stub_code_ia32.cc b/runtime/vm/stub_code_ia32.cc
index 32505f36de9041d461022829e17b5e69d6cb4769..d351ded34d5154d4c41e30a490505015dfbeba47 100644
--- a/runtime/vm/stub_code_ia32.cc
+++ b/runtime/vm/stub_code_ia32.cc
@@ -877,9 +877,9 @@ void StubCode::GenerateAllocateContextStub(Assembler* assembler) {
     // EBX: next object start.
     // EDX: number of context variables.
     __ movl(Address(ECX, Heap::TopOffset(space)), EBX);
-    __ addl(EAX, Immediate(kHeapObjectTag));
     // EBX: Size of allocation in bytes.
     __ subl(EBX, EAX);
+    __ addl(EAX, Immediate(kHeapObjectTag));
     // Generate isolate-independent code to allow sharing between isolates.
     __ UpdateAllocationStatsWithSize(cid, EBX, EDI, space,
                                      /* inline_isolate = */ false);