The TypedArray.prototype[Symbol.toStringTag] getter is currently the best (and
as far as I can tell only definitely side-effect free) way to check whether an
arbitrary object is a TypedArray - either generally TypedArray or a specific
one like Uint8Array. Using the getter is thus emerging as the general pattern
to detect TypedArrays, even Node.js now adapted it starting with

  https://github.com/nodejs/node/pull/15663

for the isTypedArray and isUint8Array type checks in lib/internal/util/types.js
now.

The getter returns either the string with the TypedArray subclass name
(i.e. "Uint8Array") or undefined if the receiver is not a TypedArray.
This can be implemented with a simple elements kind dispatch, instead of
checking the instance type and then loading the class name from the
constructor, which requires a loop walking up the transition tree. This
CL ports the builtin to CSA and TurboFan, and changes the logic to a
simple elements kind check. On the micro-benchmark mentioned in the
referenced bug, the time goes from

  testIsArrayBufferView: 565 ms.
  testIsTypedArray: 2403 ms.
  testIsUint8Array: 3847 ms.

to

  testIsArrayBufferView: 566 ms.
  testIsTypedArray: 965 ms.
  testIsUint8Array: 965 ms.

which presents an up to 4x improvement.

Bug: v8:6874
Change-Id: I9c330b4529d9631df2f052acf023c6a4fae69611
Reviewed-on: https://chromium-review.googlesource.com/695021Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48254}