fix #6093, inject should not be merged with Object.create

[jkzing]

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

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
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Does this PR introduce a breaking change? (check one)

• Yes
• No

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• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• [ x When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
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• New/updated tests are included

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Other information:
fix #6093

Object.create is not adding 'own property' to the object, which caused injections not injected properly.😭

[javoski]

Thanks for the PR.
Actually there's no need to add another merge strategy function, it's an issue about using Object.keys() to iterate inject object while using for...in on other options. (Object.keys() will not enumerates properties in the prototype chain, but for...in does).
See https://github.com/vuejs/vue/blob/dev/src/core/instance/inject.js#L21

So my suggestion is:

strats.props =
strats.methods =
strats.inject =
strats.computed = function (parentVal: ?Object, childVal: ?Object): ?Object {
  if (!parentVal) return childVal
  const ret = Object.create(null)
  extend(ret, parentVal)
  if (childVal) extend(ret, childVal)
  return ret
}

// Just delete this line will fix that issue :
if (!childVal) return Object.create(parentVal || null)

[jkzing]

@javoski You are right, I did think about that too. But I just didn't have the confident to remove that line directly. (I was afraid it's using Object.create for a reason)

Anyway, If it works totally OK without Object.create, it's better to merge your PR, and feel free to close this one when merging it.

[yyx990803]

Thanks, #6093 seems to be a simpler fix.