fix: Use correct data length in Array#flat (#2058)

Author: dcodeIO

std/assembly/array.ts

@@ -473,7 +473,8 @@ export class Array<T> {
  }
 
  // calculate the byteLength of the resulting backing ArrayBuffer
- var byteLength = <usize>size << usize(alignof<valueof<T>>());
+ const align = alignof<valueof<T>>();
+ var byteLength = <usize>size << align;
  var outBuffer = changetype<ArrayBuffer>(__new(byteLength, idof<ArrayBuffer>()));
 
  // create the return value and initialize it
@@ -492,14 +493,14 @@ export class Array<T> {
  let child = load<usize>(ptr + (<usize>i << alignof<T>()));
 
  // ignore null arrays
- if (child == 0) continue;
+ if (!child) continue;
 
  // copy the underlying buffer data to the result buffer
- let childDataLength = load<i32>(child, offsetof<T>("byteLength"));
+ let childDataLength = <usize>load<i32>(child, offsetof<T>("length_")) << align;
  memory.copy(
  changetype<usize>(outBuffer) + resultOffset,
  load<usize>(child, offsetof<T>("dataStart")),
- <usize>childDataLength
+ childDataLength
  );
 
  // advance the result length