turn transform into a simple for loop

[mattkrick]

I'm all for expressive code, but the transform function is HOT. Profiling my webpack build, the function accounts for ~20% of the ticks. This is verified through my webpack build times: I reduced my build from ~12 seconds to ~10 seconds.
Before:

After:

I'm not saying we should do this for every forEach, but a 20% reduction in build times just for changing a loop type seems like an easy win.

[codecov-io]

Current coverage is 85.81%

Merging #3477 into master will not affect coverage as of 431e77c

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  Files          197     197       
  Stmts        12227   12228     +1
  Branches      2521    2521       
  Methods          0       0       
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  Hit          10493   10493       
- Partial        522     523     +1
  Missed        1212    1212       

Review entire Coverage Diff as of 431e77c

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[loganfsmyth]

Thanks! I'm surprised it was that much of a difference. Are you using passPerPreset?

[milesj]

Native forEach() is notoriously slower than for loops, so this isn't really surprising. This could be a quick win if it was changed in areas of the code that are hit the most.

[loganfsmyth]

Totally, but I would not have expected the top level transform function to be called enough to matter, hence my surprise.

[hzoo]

We could also use for (let pluginPass of (this.pluginPasses: Array)) {} to take advantage of our Array type optimization which transpiles to a regular loop which is in rest of the codebase

[mattkrick]

@loganfsmyth nah, just a real standard config. I bet it'd do even better for those folks using passPerPreset.
I was really surprised, too, which is why I included the results (I wouldn't believe it if I didn't run it myself).

[Macil]

@hzoo Are you sure that works? An optimization like that sounds handy but I don't observe that doing anything special in the output code, even in building Babel itself.

[hzoo]

@agentme you can take that convo to #3485 (and as logan mentioned it's not special for babel since we already use loose mode - only makes the output simpler.