refactor(core): Replace "var"

[slichlyter12]

Replaces instances of "var" with "let" and "const" where applicable using the eslint 'no-var' and
'prefer-const' rules to help with the migration to ES6.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included (N/A)

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

[sodatea]

package-lock.json should be excluded from the commits, for the following reasons:

1. We already have a yarn.lock file;
2. Lockfiles should only be updated when we have to, with great caution and thorough testing.

The rest of the PR seems fine.

[slichlyter12]

I have removed package-lock.json from the commit.