Allows turning on checked mode on a per-isolate basis

runtime/vm/intrinsifier_x64.cc

Index: runtime/vm/intrinsifier_x64.cc
===================================================================
--- runtime/vm/intrinsifier_x64.cc	(revision 43472)
+++ runtime/vm/intrinsifier_x64.cc	(working copy)
@@ -33,7 +33,7 @@
 
 
 void Intrinsifier::ObjectArraySetIndexed(Assembler* assembler) {
-  if (FLAG_enable_type_checks) {
+  if (Isolate::Current()->TypeChecksEnabled()) {
     return;
   }
   __ movq(RDX, Address(RSP, + 1 * kWordSize));  // Value.
@@ -102,7 +102,7 @@
 // On stack: growable array (+2), value (+1), return-address (+0).
 void Intrinsifier::GrowableArray_add(Assembler* assembler) {
   // In checked mode we need to check the incoming argument.
-  if (FLAG_enable_type_checks) return;
+  if (Isolate::Current()->TypeChecksEnabled()) return;
   Label fall_through;
   __ movq(RAX, Address(RSP, + 2 * kWordSize));  // Array.
   __ movq(RCX, FieldAddress(RAX, GrowableObjectArray::length_offset()));