Use first binding for multiple var declarations

[peey]

Q	A
Patch: Bug Fix?	yes
Major: Breaking Change?	no
Minor: New Feature?	no
Deprecations?	no
Spec Compliancy?	yes
Tests Added/Pass?	yes
Fixed Tickets	Fixes #2378
License	MIT
Doc PR	no
Dependency Changes	no

Since var declarations after initial binding have no effect, use the first declaration (instead of last, as we were currently doing). This prevents a param binding from turning into a var binding when a var declaration of the same identifier is made in the function body. Hence it fixes #2378

[mention-bot]

@peey, thanks for your PR! By analyzing the history of the files in this pull request, we identified @existentialism and @loganfsmyth to be potential reviewers.

[codecov]

Codecov Report

Merging #5745 into 7.0 will decrease coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##              7.0   #5745      +/-   ##
=========================================
- Coverage   84.63%   84.6%   -0.04%     
=========================================
  Files         282     282              
  Lines        9861    9864       +3     
  Branches     2766    2767       +1     
=========================================
- Hits         8346    8345       -1     
- Misses        997    1001       +4     
  Partials      518     518

Impacted Files	Coverage Δ
packages/babel-traverse/src/scope/binding.js	75.6% <100%> (+1.92%)	⬆️
packages/babel-helper-call-delegate/src/index.js	64% <0%> (-4%)	⬇️
packages/babel-traverse/src/path/modification.js	73.78% <0%> (-0.98%)	⬇️
...bel-plugin-transform-es2015-classes/src/vanilla.js	90.12% <0%> (-0.86%)	⬇️
packages/babel-traverse/src/path/context.js	86.2% <0%> (ø)	⬆️

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[jridgewell]

Nit: Is this still necessary?
https://github.com/babel/babel/pull/5745/files#diff-739bf54b7930434d653e3e1bbb701154R37?

[peey]

@jridgewell I forgot. I'll have to check.

Also, this shouldn't be merged yet, I just noticed an error in one of the tests: this should also have been renamed to z

I'll patch it up and update here

[peey]

@jridgewell I don't think we need it, so I have removed it. The reason being that there are only two categories of redeclarations - one we're fine with and one that result in an error.

The redeclarations which result in an error are taken care of here

And rest of them (like var on param or hoisted on var, etc) result in creation of a new Binding where the code in this PR will take care of the bindings.