VM: Make use_osr an Isolate flag, similar to how we made use_field_guards.

runtime/vm/intermediate_language_x64.cc

Index: runtime/vm/intermediate_language_x64.cc
diff --git a/runtime/vm/intermediate_language_x64.cc b/runtime/vm/intermediate_language_x64.cc
index 0193a09ce0cc9547cff3758a81abffeb1f95d576..c704319b8712dc174d92c958548fed0c6f7863b5 100644
--- a/runtime/vm/intermediate_language_x64.cc
+++ b/runtime/vm/intermediate_language_x64.cc
@@ -2632,7 +2632,7 @@ class CheckStackOverflowSlowPath : public SlowPathCode {
       : instruction_(instruction) {}
 
   virtual void EmitNativeCode(FlowGraphCompiler* compiler) {
-    if (FLAG_use_osr && osr_entry_label()->IsLinked()) {
+    if (compiler->isolate()->use_osr() && osr_entry_label()->IsLinked()) {
       __ Comment("CheckStackOverflowSlowPathOsr");
       __ Bind(osr_entry_label());
       __ movq(Address(THR, Thread::stack_overflow_flags_offset()),
@@ -2650,7 +2650,8 @@ class CheckStackOverflowSlowPath : public SlowPathCode {
         instruction_->token_pos(), instruction_->deopt_id(),
         kStackOverflowRuntimeEntry, 0, instruction_->locs());
 
-    if (FLAG_use_osr && !compiler->is_optimizing() && instruction_->in_loop()) {
+    if (compiler->isolate()->use_osr() && !compiler->is_optimizing() &&
+        instruction_->in_loop()) {
       // In unoptimized code, record loop stack checks as possible OSR entries.
       compiler->AddCurrentDescriptor(RawPcDescriptors::kOsrEntry,
                                      instruction_->deopt_id(),
@@ -2663,7 +2664,7 @@ class CheckStackOverflowSlowPath : public SlowPathCode {
 
 
   Label* osr_entry_label() {
-    ASSERT(FLAG_use_osr);
+    ASSERT(Isolate::Current()->use_osr());
     return &osr_entry_label_;
   }