Add one-char string table for faster String.charAt to the VM.

runtime/lib/string.cc

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/bootstrap_natives.h"
 
 #include "vm/exceptions.h"
 #include "vm/native_entry.h"
 #include "vm/object.h"
 #include "vm/symbols.h"
@@ skipping 84 matching lines @@
     return 0;
   }
 }
 
 
 DEFINE_NATIVE_ENTRY(String_charAt, 2) {
   const String& receiver = String::CheckedHandle(arguments->At(0));
   GET_NATIVE_ARGUMENT(Integer, index, arguments->At(1));
   uint32_t value = StringValueAt(receiver, index);
   ASSERT(value <= 0x10FFFF);
-  return Symbols::New(&value, 1);
+  return Symbols::FromCharCode(value);
 }
 
 DEFINE_NATIVE_ENTRY(String_charCodeAt, 2) {
   const String& receiver = String::CheckedHandle(arguments->At(0));
   GET_NATIVE_ARGUMENT(Integer, index, arguments->At(1));
   int32_t value = StringValueAt(receiver, index);
   ASSERT(value >= 0);
   ASSERT(value <= 0x10FFFF);
   return Smi::New(value);
 }
@@ skipping 34 matching lines @@
     if (!elem.IsString()) {
       GrowableArray<const Object*> args;
       args.Add(&elem);
       Exceptions::ThrowByType(Exceptions::kArgument, args);
     }
   }
   return String::ConcatAll(strings);
 }
 
 }  // namespace dart