assert: refactor to avoid unsafe array iteration

[aduh95]

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

[aduh95]

These changes come with some perf regressions:

                                                   confidence improvement accuracy (*)    (**)   (***)
 assert/ok.js n=100000                                    ***     -6.77 %       ±3.69%  ±4.92%  ±6.44%
 assert/throws.js method='doesNotThrow' n=10000           ***    -14.90 %       ±5.67%  ±7.56%  ±9.86%

This may or may not be considered an acceptable trade-off.

[aduh95]

#37005 should help solve the perf issue.

EDIT: Nope, it doesn't help.