Fix readable/writable file stream examples

[dubiousjim]

These examples weren't closing the filehandle correctly (they called close on a nonexistent fd variable, rather than calling the fileHandle.close method).

Also the ReadableStream example misused the fileHandle.read call: it was supplying an ArrayBuffer and v's byteOffset within the ArrayBuffer, but the API requires supplying v itself and an offset relative to v's start.

Also added the file offset position to that fileHandle.read call. There's no reason otherwise for the example to keep track of this. Alternatively, the argument could be supplied as null, and we just rely on implicit file positioning.

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Preview | Diff

[MattiasBuelens]

Thanks! Looks good. 👍

Also the ReadableStream example misused the fileHandle.read call: it was supplying an ArrayBuffer and v's byteOffset within the ArrayBuffer, but the API requires supplying v itself and an offset relative to v's start.

I noticed a similar mistake in the "A readable stream with an underlying pull source" example. We're passing an ArrayBuffer to fileHandle.read(), but it only accepts Buffer | TypedArray | DataView.

streams/index.bs

Lines 7305 to 7307 in 5885d94

	const buffer = new ArrayBuffer(CHUNK_SIZE);
	const { bytesRead } = await fileHandle.read(buffer, 0, CHUNK_SIZE, position);

We should probably use new Uint8Array(CHUNK_SIZE) instead, and use .subarray(0, bytesRead) (or .slice) in the controller.enqueue() call. Could you fix that too as part of this PR? 🙂