fixup! do not use writer public API

Author: MattiasBuelens

lib/internal/webstreams/readablestream.js

@@ -127,6 +127,7 @@ const {
   writableStreamAbort,
   writableStreamCloseQueuedOrInFlight,
   writableStreamDefaultWriterCloseWithErrorPropagation,
+  writableStreamDefaultWriterGetDesiredSize,
   writableStreamDefaultWriterRelease,
   writableStreamDefaultWriterWrite,
 } = require('internal/webstreams/writablestream');
@@ -1564,7 +1565,7 @@ function readableStreamPipeTo(
 
         // Check backpressure after each write
         if (dest[kState].state === 'writable') {
-          const desiredSize = writer.desiredSize;
+          const desiredSize = writableStreamDefaultWriterGetDesiredSize(writer);
           if (desiredSize !== null && desiredSize <= 0) {
             // Backpressure - stop batch and wait for ready
             break;