fix(provide): Merges symbol provides

[privatenumber]

Fixes merging multiple provides using Symbols

fix #7923

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

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:
The test I added only passes in an environment with symbols (since it's handling a case that fails with symbols). This fails when the test is executed with Phantom, since it doesn't have Symbol support natively or with a polyfill, but passes in Chrome.

I can add babel-polyfill but seemed excessive for one test. Wanted to know how to move forward with that one before making any drastic changes to making it work.

[privatenumber]

Wasn't entirely sure if this was needed but took it from inject.js to be safe.

[posva]

Thanks for the contribution and sorry for the delay! I moved the test so it isn't run if Reflect isn't supported

[privatenumber]

@posva Oh nice, thanks!

[yyx990803]

Note: Reflect.ownKeys + filter with descriptor check is about 10x slower than Object.keys(), I think we need to avoid this cost for normal data merging, and only do this for provide() merges.