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

runtime/vm/symbols.cc

Index: runtime/vm/symbols.cc
diff --git a/runtime/vm/symbols.cc b/runtime/vm/symbols.cc
index d7afc33b32bee0558a598664a3ae2568b501a51d..3fcba6f8cf201285ed4107ea71adfe3ecebbbb0b 100644
--- a/runtime/vm/symbols.cc
+++ b/runtime/vm/symbols.cc
@@ -383,7 +383,13 @@ RawString* Symbols::FromUTF32(const int32_t* utf32_array, intptr_t 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));
+  }
 }