babel-jest is incompatible with plugins made for Babel 7

[bradchristensen]

Do you want to request a feature or report a bug?
Bug

What is the current behavior?
When using babel-jest in a project using Babel 7, babel-jest still attempts to transform code using babel-core@6 (because it is specified as a dependency rather than a peerDependency), which is not necessarily compatible with the plugins/presets that the user will have included in the project.

For example, I have a project using Babel 7 and the babel-plugin-transform-object-rest-spread Babel plugin. If I have version 7.0.0-alpha.15 installed, babel-jest behaves as if the plugin is not referenced at all, throwing the error:

    function ProfileLabel({ className, ...props }) {
                                       ^^^

    SyntaxError: Unexpected token ...

However, if the version of babel-core installed as a dependency of babel-jest is changed to version 7.0.0-alpha.15, the plugin is used correctly and the code is transpiled as expected.

What is the expected behavior?

It seems to me that the obvious solution to this would be to change babel-core to be a peerDependency instead of a dependency, since any projects that rely on babel-jest presumably have to have babel-core installed anyway. Even though this would be a breaking change it would maintain backwards compatibility with Babel 6. I have tested and confirmed on my machine that modifying the package.json as follows fixes the issue.

{
  ...
  "dependencies": {
    "babel-plugin-istanbul": "^4.0.0",
    "babel-preset-jest": "^20.0.3"
  },
  "devDependencies": {
    "babel-core": "^7.0.0-alpha.15"
  },
  "peerDependencies": {
    "babel-core": "^6.0.0 || ^7.0.0-alpha || ^7.0.0"
  }
}

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