Remove new space allocation in optimizing compiler, use Symbols::FromConcat where appropriate

runtime/vm/scanner.cc

Index: runtime/vm/scanner.cc
diff --git a/runtime/vm/scanner.cc b/runtime/vm/scanner.cc
index 3e7f02834cf327eb420aa2982f5a791d3ce426d9..d5533eb51344124c239ef28af787c1d133f9ac24 100644
--- a/runtime/vm/scanner.cc
+++ b/runtime/vm/scanner.cc
@@ -331,8 +331,7 @@ void Scanner::ScanIdentChars(bool allow_dollar) {
       String::ZoneHandle(Z, Symbols::New(source_, ident_pos, ident_length));
   if (ident_char0 == Library::kPrivateIdentifierStart) {
     // Private identifiers are mangled on a per library basis.
-    literal = String::Concat(literal, private_key_);
-    literal = Symbols::New(literal);
+    literal = Symbols::FromConcat(literal, private_key_);
   }
   current_token_.literal = &literal;
 }