update babel config to target newer browsers, work around parcel bug

[tafsiri]

This does a few things.

It changes the build target for all the examples to support the following newer browsers currently,

and_chr 64
and_uc 11.8
chrome 64
chrome 63
firefox 58
ios_saf 11.0-11.2

The exception to this is getting_started, which still doesn't use a build tool, and webcam-transfer-learning. The reason for the latter is because that demo is featured on our front page it is more likely to be viewed by non-developers, who may have older browsers.

Targetting newer browsers makes the stack traces easier to parse as mentioned in tensorflow/tfjs#60

The other thing this PR does is workaround a bug in parcel that was often causing an error to happen in the dev server and require a restart of yarn. Moving to .babelrc files should fix the problem and allow for much smoother development.

I also updated the readme to spell out the dependencies needed, which should close tensorflow/tfjs#57

This one might be nice for people to try out locally.

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This change is 

[dsmilkov]

couple small comments

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Reviewed 24 of 24 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved.

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README.md, line 13 at r1 (raw file):

Except for `getting_started`, all the examples require the following dependencies to be installed.

 - Node.js version 8.9 or higher

curious, where does 8.9 come from? Is that parcel's requirement?

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mnist-core/dist-orig/index.html, line 1 at r1 (raw file):

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shouldn't this file live directly under mnist-core and not in a dist-orig folder?

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Comments from Reviewable

[tafsiri]

Review status: all files reviewed at latest revision, 2 unresolved discussions, all commit checks successful.

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README.md, line 13 at r1 (raw file):

Previously, dsmilkov (Daniel Smilkov) wrote…

curious, where does 8.9 come from? Is that parcel's requirement?

Yep this was working around a bug in parcel on older versions of node. 8.9 is the first LTS release of node after node 6 (the prev LTS release)

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mnist-core/dist-orig/index.html, line 1 at r1 (raw file):

Previously, dsmilkov (Daniel Smilkov) wrote…

shouldn't this file live directly under mnist-core and not in a dist-orig folder?

Good catch. this file should not even be here. I was diffing against the new output. will delete.

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Comments from Reviewable

[nsthorat]

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Review status: all files reviewed at latest revision, 2 unresolved discussions, all commit checks successful.

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Comments from Reviewable