Convert all symbols accessor to return read only handles so that it is not necessary to create a ne…

vm/scanner.cc

Index: vm/scanner.cc
===================================================================
--- vm/scanner.cc	(revision 16583)
+++ vm/scanner.cc	(working copy)
@@ -426,19 +426,16 @@
     SkipLine();
     return;
   }
-  const String& kLibrary = String::Handle(Symbols::Library());
-  const String& kImport = String::Handle(Symbols::Import());
-  const String& kSource = String::Handle(Symbols::Source());
   const String& ident = String::Handle(ConsumeIdentChars(false));
-  if (ident.Equals(kLibrary)) {
+  if (ident.Equals(Symbols::Library())) {
     current_token_.kind = Token::kLEGACY_LIBRARY;
     return;
   }
-  if (ident.Equals(kImport)) {
+  if (ident.Equals(Symbols::Import())) {
     current_token_.kind = Token::kLEGACY_IMPORT;
     return;
   }
-  if (ident.Equals(kSource)) {
+  if (ident.Equals(Symbols::Source())) {
     current_token_.kind = Token::kLEGACY_SOURCE;
     return;
   }