Renamed "symbols" to "internalized strings" throughout the code base,

src/execution.cc

Index: src/execution.cc
diff --git a/src/execution.cc b/src/execution.cc
index fa4116730aaa82ebe811510fef479271fd5345ba..5a991cabb11ed8df8686f98ddb4c8e459f6b0385 100644
--- a/src/execution.cc
+++ b/src/execution.cc
@@ -683,7 +683,7 @@ Handle<Object> Execution::CharAt(Handle<String> string, uint32_t index) {
 
   Handle<Object> char_at =
       GetProperty(isolate->js_builtins_object(),
-                  factory->char_at_symbol());
+                  factory->char_at_string());
   if (!char_at->IsJSFunction()) {
     return factory->undefined_value();
   }
@@ -784,7 +784,7 @@ Handle<String> Execution::GetStackTraceLine(Handle<Object> recv,
                                   args,
                                   &caught_exception);
   if (caught_exception || !result->IsString()) {
-      return isolate->factory()->empty_symbol();
+      return isolate->factory()->empty_string();
   }
 
   return Handle<String>::cast(result);