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

runtime/vm/symbols.cc

 // Copyright (c) 2012, 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/symbols.h"
 
 #include "vm/handles.h"
 #include "vm/handles_impl.h"
 #include "vm/hash_table.h"
 #include "vm/isolate.h"
@@ skipping 365 matching lines @@
   return NewSymbol(UTF16Array(utf16_array, len));
 }
 
 
 RawString* Symbols::FromUTF32(const int32_t* utf32_array, intptr_t len) {
   return NewSymbol(UTF32Array(utf32_array, len));
 }
 
 
 RawString* Symbols::FromConcat(const String& str1, const String& str2) {
-  return NewSymbol(ConcatString(str1, str2));
+  if (str1.Length() == 0) {
+    return New(str2);
+  } else if (str2.Length() == 0) {
+    return New(str1);
+  } else {
+    return NewSymbol(ConcatString(str1, str2));
+  }
 }
 
 
 // StringType can be StringSlice, ConcatString, or {Latin1,UTF16,UTF32}Array.
 template<typename StringType>
 RawString* Symbols::NewSymbol(const StringType& str) {
   Thread* thread = Thread::Current();
   Isolate* isolate = thread->isolate();
   Zone* zone = thread->zone();
   String& symbol = String::Handle(zone);
@@ skipping 66 matching lines @@
 RawObject* Symbols::GetVMSymbol(intptr_t object_id) {
   ASSERT(IsVMSymbolId(object_id));
   intptr_t i = (object_id - kMaxPredefinedObjectIds);
   if ((i > kIllegal) && (i < Symbols::kMaxPredefinedId)) {
     return symbol_handles_[i]->raw();
   }
   return Object::null();
 }
 
 }  // namespace dart