Allows turning on checked mode on a per-isolate basis

runtime/vm/intrinsifier_ia32.cc

Index: runtime/vm/intrinsifier_ia32.cc
===================================================================
--- runtime/vm/intrinsifier_ia32.cc	(revision 43426)
+++ runtime/vm/intrinsifier_ia32.cc	(working copy)
@@ -49,7 +49,7 @@
 // update. Array length is always a Smi.
 void Intrinsifier::ObjectArraySetIndexed(Assembler* assembler) {
   Label fall_through;
-  if (FLAG_enable_type_checks) {
+  if (Isolate::Current()->TypeChecksEnabled()) {
     const intptr_t type_args_field_offset =
         ComputeObjectArrayTypeArgumentsOffset();
     // Inline simple tests (Smi, null), fallthrough if not positive.
@@ -150,7 +150,7 @@
 // On stack: growable array (+2), value (+1), return-address (+0).
 void Intrinsifier::GrowableArray_add(Assembler* assembler) {
   // In checked mode we need to type-check the incoming argument.
-  if (FLAG_enable_type_checks) return;
+  if (Isolate::Current()->TypeChecksEnabled()) return;
 
   Label fall_through;
   __ movl(EAX, Address(ESP, + 2 * kWordSize));  // Array.