Bindings generation emits (more) correct null checking

Source/bindings/dart/scripts/templates/interface_h.template

Index: Source/bindings/dart/scripts/templates/interface_h.template
diff --git a/Source/bindings/dart/scripts/templates/interface_h.template b/Source/bindings/dart/scripts/templates/interface_h.template
index 353d0883faa9401b9b3ea4f4c69dcc670122f979..8d5f4a233a4c5dd4506e4fb569eb1c57dd665775 100644
--- a/Source/bindings/dart/scripts/templates/interface_h.template
+++ b/Source/bindings/dart/scripts/templates/interface_h.template
@@ -57,26 +57,42 @@ struct {{dart_class}} {
         return static_cast<{{cpp_class}}*>(value);
     }
 
+    // FIXME: This has redundant null checks, and much of this code should
+    // be moved to a common place to reduce code bloat.
+    // Convert to native, Dart_Null is an error
     static NativeType* toNative(Dart_Handle handle, Dart_Handle& exception)
     {
-        DartDOMData* domData = DartDOMData::current();
-        return DartDOMWrapper::unwrapDartWrapper<{{dart_class}}>(domData, handle, exception);
+        if (!Dart_IsNull(handle)) {
+            DartDOMData* domData = DartDOMData::current();
+            return DartDOMWrapper::unwrapDartWrapper<{{dart_class}}>(domData, handle, exception);
+        } else {
+            exception = Dart_NewStringFromCString("Null value is not a valid {{dart_class}}");
+            return 0;
+        }
     }
 
+    // Convert nullable to native, Dart_Null maps to C++ null
     static NativeType* toNativeWithNullCheck(Dart_Handle handle, Dart_Handle& exception)
     {
-        return Dart_IsNull(handle) ? 0 : toNative(handle, exception);
+        DartDOMData* domData = DartDOMData::current();
+        return DartDOMWrapper::unwrapDartWrapper<{{dart_class}}>(domData, handle, exception);
     }
 
+    // Convert to native, Dart_Null is an error
     static NativeType* toNative(Dart_NativeArguments args, int index, Dart_Handle& exception)
     {
-        return DartDOMWrapper::unwrapDartWrapper<{{dart_class}}>(args, index, exception);
+        NativeType* result = DartDOMWrapper::unwrapDartWrapper<{{dart_class}}>(args, index, exception);
+        if (result || exception) {
+            return result;
+        }
+        exception = Dart_NewStringFromCString("Null value is not a valid {{dart_class}}");
+        return 0;
     }
 
+    // Convert nullable to native, Dart_Null maps to C++ null
     static NativeType* toNativeWithNullCheck(Dart_NativeArguments args, int index, Dart_Handle& exception)
     {
-        // toNative accounts for Null objects also.
-        return toNative(args, index, exception);
+        return DartDOMWrapper::unwrapDartWrapper<{{dart_class}}>(args, index, exception);
     }
 
     static Dart_Handle toDart(NativeType* value)