Preventing client.link from being publicly mutatable

[ismail-syed]

Context

We built some abstractions on top of apollo links and noticed that mutating client.link property on an instance of ApolloClient does not actually propagate the link changes properly to the queryManager.

If this isn't how the client.links API is meant to be used, should we consider updating the client.link property to be a read-only property similar to the queryManager. This would help prevent folks from incorrectly using the API. Happy to PR if this is something you're interested in!

Intended outcome:

• If mutating client.link the new links added should work.

Actual outcome:

• If mutating client.link the new links added do not work.

How to reproduce the issue:

• ismail-syed/react-apollo-error-template@b6d1101

Versions

$ npx envinfo@latest --preset apollo --clipboard                                                                                                                                                                                      130 ↵
npx: installed 1 in 1.705s

  System:
    OS: macOS 10.14.4
  Binaries:
    Node: 12.3.1 - /usr/local/bin/node
    Yarn: 1.10.1 - /usr/local/bin/yarn
    npm: 6.9.0 - /usr/local/bin/npm
  Browsers:
    Chrome: 76.0.3809.132
    Firefox: 66.0.5
    Safari: 12.1
  npmPackages:
    apollo-cache-inmemory: ^1.6.3 => 1.6.3
    apollo-client: ^2.6.4 => 2.6.4
    apollo-link: ^1.2.12 => 1.2.12

[hwillson]

Changes to allow this were introduced in #6193 - thanks!