[dart2wasm] Mark _typeArguments as synthetic

Fixes b/293426600

This synthetic procedure is added on every class with type args.
Mark it as synthetic so the JS interop checks don't trigger on
it.

Change-Id: I5b04fe4340b409a4ab1eb5c32e37fe6ff37df4b0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/316702
Reviewed-by: Joshua Litt <[email protected]>
Commit-Queue: Joshua Litt <[email protected]>
Auto-Submit: Srujan Gaddam <[email protected]>

pkg/dart2wasm/lib/transformers.dart

@@ -121,6 +121,7 @@ class _WasmTransformer extends Transformer {
                 Nullability.nonNullable, [_nonNullableTypeType]),
           ),
           isExternal: true,
+          isSynthetic: true,
           fileUri: cls.fileUri)
         ..isNonNullableByDefault = true;
       cls.addProcedure(getTypeArguments);