tweak: let array index less than (2 ** 32 - 1) to fit Standard ECMA

[wujieZ]

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

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe: let array index less than Math.pow(2, 32) - 1 to fit Standard ECMA

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

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:

[posva]

What is this improving exactly? Is there a case where current version wasn't working?

[wujieZ]

What is this improving exactly? Is there a case where current version wasn't working?

Thanks, Current version working no problem, but if val more than Math.pow(2, 32) - 1, it always return true, this is not fit Standard ECMA, look this http://www.ecma-international.org/ecma-262/6.0/index.html#sec-array-exotic-objects