Initial ArrayBuffer implementation; Conditional allocation within constructors; Explicit constructor return values

std/assembly/arraybuffer.ts

@@ -0,0 +1,29 @@
+const HEADER_SIZE: usize = sizeof<i32>();
+
+@unmanaged
+export class ArrayBuffer {
+
+  readonly byteLength: i32;
+
+  constructor(length: i32) {
+    if (<u32>length > 0x7fffffff) {
+      throw new RangeError("Invalid array buffer length");
+    }
+    var buffer = allocate_memory(HEADER_SIZE + <usize>length);
+    store<i32>(buffer, length);
+    return changetype<ArrayBuffer>(buffer);
+  }
+
+  slice(begin: i32 = 0, end: i32 = 0x7fffffff): ArrayBuffer {
+    var len = this.byteLength;
+    if (begin < 0) begin = max(len + begin, 0);
+    else begin = min(begin, len);
+    if (end < 0) end = max(len + end, 0);
+    else end = min(end, len);
+    var newLen = max(end - begin, 0);
+    var buffer = allocate_memory(HEADER_SIZE + <usize>newLen);
+    store<i32>(buffer, newLen);
+    move_memory(buffer + HEADER_SIZE, changetype<usize>(this) + HEADER_SIZE + begin, newLen);
+    return changetype<ArrayBuffer>(buffer);
+  }
+}