Migrate most uses of Isolate::current_zone to Thread::zone.

runtime/vm/flow_graph_compiler.h

Index: runtime/vm/flow_graph_compiler.h
diff --git a/runtime/vm/flow_graph_compiler.h b/runtime/vm/flow_graph_compiler.h
index 17b2612f0f98ec44f182135225141d2af8399ae5..2ea19ee1538fd2be0fb4dc5a28e96a0fbcfacd8a 100644
--- a/runtime/vm/flow_graph_compiler.h
+++ b/runtime/vm/flow_graph_compiler.h
@@ -177,7 +177,7 @@ class CompilerDeoptInfoWithStub : public CompilerDeoptInfo {
     const char* kFormat = "Deopt stub for id %d, reason: %s";
     const intptr_t len = OS::SNPrint(NULL, 0, kFormat,
         deopt_id(), DeoptReasonToCString(reason())) + 1;
-    char* chars = Isolate::Current()->current_zone()->Alloc<char>(len);
+    char* chars = Thread::Current()->zone()->Alloc<char>(len);
     OS::SNPrint(chars, len, kFormat,
         deopt_id(), DeoptReasonToCString(reason()));
     return chars;