fix: Observer> trouble with observe Array (losted special methods)

[cybermerlin]

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

• Bugfix -------->> and for 2.5.17 in branch hotfix_observe_array
• 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

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:

[windyflying]

@cybermerlin arrayMethods is an instance of Array.prototype. We redefined some methods of arrayMethods (such as push,splice,unshift ,etc.) in order to notify change.

[cybermerlin]

@windyflying thx, I lost it before, but now fix =)

[cybermerlin]

same decision in PR #9136

[posva]

Repeating myself:

I feel like you have rushed this pr. If it's implementing that feature it would require proper testing of the actual feature request. Opening a pr with just Hotfix as a title is so
Confusing for us maintainers...

This one isn't clear at all either

You want to contribute to Open Source, that's great, but don't try to rush it like a feature on Friday evening. We need a runnable repro as @Justineo said for bug fixes

[cybermerlin]

somebody help me with this #9137