Port more String and HashMap usages

sky/engine/tonic/dart_exception_factory.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SKY_ENGINE_TONIC_DART_EXCEPTION_FACTORY_H_
 #define SKY_ENGINE_TONIC_DART_EXCEPTION_FACTORY_H_
 
+#include <string>
+
 #include "dart/runtime/include/dart_api.h"
 #include "sky/engine/tonic/dart_persistent_value.h"
-#include "sky/engine/wtf/text/WTFString.h"
 
 namespace blink {
 class DartState;
 
 class DartExceptionFactory {
  public:
   explicit DartExceptionFactory(DartState* dart_state);
   ~DartExceptionFactory();
 
   Dart_Handle CreateNullArgumentException(int index);
-  Dart_Handle CreateException(const String& class_name, const String& message);
+  Dart_Handle CreateException(const std::string& class_name,
+                              const std::string& message);
 
  private:
   DartState* dart_state_;
   DartPersistentValue core_library_;
 };
 
 }  // namespace blink
 
 #endif  // SKY_ENGINE_TONIC_DART_EXCEPTION_FACTORY_H_