Normative: Swap side effect and check in TypedArray constructor

[littledan]

Previously, in one of the TypedArray constructor paths, an ArrayBuffer
was checked for being detached, and then a ToIndex calculation is done.
Because ToIndex may detach the ArrayBuffer itself, this patch moves
the detached check to afterwards, so that it can include that possible
time when the ArrayBuffer may be detached.

Closes #842

[littledan]

@psmarshall Is this what you were thinking?

[psmarshall]

Yes that looks good to me, thanks

[leobalter]

this reached consensus in March 22nd at the last TC39 meeting.

[anba]

Two questions:

• The proposed changes don't match the test case (test/built-ins/TypedArrays/buffer-arg-byteoffset-to-number-detachbuffer.js). Did you also mean to move step 7 (If offset modulo elementSize ≠ 0, throw a RangeError exception.) after step 9 (If IsDetachedBuffer(buffer) is true, throw a TypeError exception.), or is the test case incorrect?
• We should also change the DataView constructor, shouldn't we? (The DataView constructor also calls ToIndex after checking for a detached array buffer.)

[littledan]

@anba

• Oops, this seems like a test bug.
• Yes, that sounds like a good idea.