Cleanup dart to string conversions.

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

Index: Source/WebCore/bindings/dart/custom/DartHTMLCanvasElementCustom.cpp
diff --git a/Source/WebCore/bindings/dart/custom/DartHTMLCanvasElementCustom.cpp b/Source/WebCore/bindings/dart/custom/DartHTMLCanvasElementCustom.cpp
index c5df422def13801374af1ab39b6bfcbc6b91a9b2..5b2c91c46e368a5a2ff7e0c00bbf0c2ea7eb4b15 100644
--- a/Source/WebCore/bindings/dart/custom/DartHTMLCanvasElementCustom.cpp
+++ b/Source/WebCore/bindings/dart/custom/DartHTMLCanvasElementCustom.cpp
@@ -56,11 +56,9 @@ void toDataURLCallback(Dart_NativeArguments args)
     {
         HTMLCanvasElement* receiver = DartDOMWrapper::receiver<HTMLCanvasElement>(args);
 
-        const ParameterAdapter<String> type(Dart_GetNativeArgument(args, 1));
-        if (!type.conversionSuccessful()) {
-            exception = type.exception();
+        DartStringAdapter type = DartUtilities::dartToString(Dart_GetNativeArgument(args, 1), exception);
+        if (exception)
             goto fail;
-        }
 
         ExceptionCode ec = 0;
         String result = receiver->toDataURL(type, 0, ec);
@@ -85,11 +83,9 @@ void getContextCallback(Dart_NativeArguments args)
     {
         HTMLCanvasElement* receiver = DartDOMWrapper::receiver<HTMLCanvasElement>(args);
 
-        const ParameterAdapter<String> contextId(Dart_GetNativeArgument(args, 1));
-        if (!contextId.conversionSuccessful()) {
-            exception = contextId.exception();
+        DartStringAdapter contextId = DartUtilities::dartToString(Dart_GetNativeArgument(args, 1), exception);
+        if (exception)
             goto fail;
-        }
 
         RefPtr<CanvasContextAttributes> attrs;
 #if ENABLE(WEBGL)