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

runtime/vm/disassembler_ia32.cc

Index: runtime/vm/disassembler_ia32.cc
diff --git a/runtime/vm/disassembler_ia32.cc b/runtime/vm/disassembler_ia32.cc
index c63057b80987ea1f5ebf98936fe5d7353b5d0f7c..852621bf5fe488619626458a03a180a72e129e1d 100644
--- a/runtime/vm/disassembler_ia32.cc
+++ b/runtime/vm/disassembler_ia32.cc
@@ -481,7 +481,7 @@ static const char* ObjectToCStringNoGC(const Object& obj) {
   const char* full_class_name = clazz.ToCString();
   const char* format = "instance of %s";
   intptr_t len = OS::SNPrint(NULL, 0, format, full_class_name) + 1;
-  char* chars = Isolate::Current()->current_zone()->Alloc<char>(len);
+  char* chars = Thread::Current()->zone()->Alloc<char>(len);
   OS::SNPrint(chars, len, format, full_class_name);
   return chars;
 }