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

runtime/vm/scanner.cc

Index: runtime/vm/scanner.cc
diff --git a/runtime/vm/scanner.cc b/runtime/vm/scanner.cc
index 19509b5188b1917121ae9595b91f920ab4c46813..4b1e550db421ed796b2955c1606626de8c81174f 100644
--- a/runtime/vm/scanner.cc
+++ b/runtime/vm/scanner.cc
@@ -332,7 +332,7 @@ void Scanner::ScanIdentChars(bool allow_dollar) {
   String& literal = String::ZoneHandle(Z);
   if (ident_char0 == Library::kPrivateIdentifierStart) {
     // Private identifiers are mangled on a per library basis.
-    literal = String::SubString(source_, ident_pos, ident_length);
+    literal = String::SubString(T, source_, ident_pos, ident_length);
     literal = Symbols::FromConcat(T, literal, private_key_);
   } else {
     literal = Symbols::New(T, source_, ident_pos, ident_length);