function mapArrayBufferViews(ary) {
	  return ary.map(function(chunk) {
	    if (chunk.buffer instanceof ArrayBuffer) {
	      var buf = chunk.buffer;
	
	      // if this is a subarray, make a copy so we only
	      // include the subarray region from the underlying buffer
	      if (chunk.byteLength !== buf.byteLength) {
	        var copy = new Uint8Array(chunk.byteLength);
	        copy.set(new Uint8Array(buf, chunk.byteOffset, chunk.byteLength));
	        buf = copy.buffer;
	      }
	
	      return buf;
	    }
	
	    return chunk;
	  });
	}
Helper function that maps ArrayBufferViews to ArrayBuffers
Used by BlobBuilder constructor and old browsers that didn't
support it in the Blob constructor.