Use a reusable handle in SubString instead of creating one as this path is now used when creating S…

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index 33cb344bd4698ea7c22bb47507b844880f86de3f..02b05d549558c105ea73c78f47e940c39a3a1766 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -20050,7 +20050,8 @@ RawString* String::SubString(const String& str,
 }
 
 
-RawString* String::SubString(const String& str,
+RawString* String::SubString(Thread* thread,
+                             const String& str,
                              intptr_t begin_index,
                              intptr_t length,
                              Heap::Space space) {
@@ -20063,7 +20064,6 @@ RawString* String::SubString(const String& str,
   if (begin_index > str.Length()) {
     return String::null();
   }
-  String& result = String::Handle();
   bool is_one_byte_string = true;
   intptr_t char_size = str.CharSize();
   if (char_size == kTwoByteChar) {
@@ -20074,6 +20074,8 @@ RawString* String::SubString(const String& str,
       }
     }
   }
+  REUSABLE_STRING_HANDLESCOPE(thread);
+  String& result = thread->StringHandle();
   if (is_one_byte_string) {
     result = OneByteString::New(length, space);
   } else {