Cleanup dart to string conversions.

Source/WebCore/bindings/dart/custom/DartWebKitCSSMatrixCustom.cpp

 // Copyright 2011, Google Inc.
 // All rights reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //    * Redistributions of source code must retain the above copyright
 // notice, this list of conditions and the following disclaimer.
 //    * Redistributions in binary form must reproduce the above
@@ skipping 26 matching lines @@
 
 namespace WebCore {
 
 namespace DartWebKitCSSMatrixInternal {
 
 void constructorCallback(Dart_NativeArguments args)
 {
     DartApiScope dartApiScope;
     Dart_Handle exception = 0;
     {
-        const ParameterAdapter<String> cssValue(Dart_GetNativeArgument(args, 0), DartUtilities::ConvertNullToDefaultValue);
-        if (!cssValue.conversionSuccessful()) {
-            exception = cssValue.exception();
+        DartStringAdapter cssValue = DartUtilities::dartToStringWithNullCheck(Dart_GetNativeArgument(args, 0), exception);
+        if (exception)
             goto fail;
-        }
 
         ExceptionCode ec = 0;
         RefPtr<WebKitCSSMatrix> matrix = WebKitCSSMatrix::create(cssValue, ec);
         if (UNLIKELY(ec)) {
             exception = DartDOMWrapper::exceptionCodeToDartException(ec);
             goto fail;
         }
 
         DartDOMWrapper::returnValue<DartWebKitCSSMatrix>(args, matrix.release());
         return;
     }
 
 fail:
     Dart_ThrowException(exception);
     ASSERT_NOT_REACHED();
 }
 
 } // DartWebKitCSSMatrixInternal
 
 } // WebCore