fix props & computed proto

[LongTengDao]

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:

Key in Object.prototype does not work right for props / computed before this PR.

NOTICE: here still leave constructor that can't use, even after I replaced all constructor to _constructor in the source code, if I remove line Vue.prototype.constructor = Vue (in /src/core/instance/index.js) for the last step, here will always be one test fail (ci/circleci: test-e2e). I can not figure out why:

   - basic SSR (664ms)
   Testing if element <#result> contains text: "<div data-server-rendered="true">foo</div>".  - expected "<div data-server-rendered="true">foo</div>" but got: "TypeError: undefined is not an object (evaluating 'Vue.super')"
       at Object.basic SSR (/root/project/vue/test/e2e/specs/basic-ssr.js:5:15)
       at processTicksAndRejections (internal/process/task_queues.js:79:11)

[posva]

Thank you although there doesn't seem to be a problem here. Maybe you should open an bug report with a reproduction first