DEV_SSR true does not seem to work with no trailing slashes

[iDVB]

Description

The new DEV_SSR: true flag does not seem to work properly when testing without trailing slashes.

Using DEV_SSR it seems that gatsby links cannot properly determine isCurrent the same way between the client and server if you do not have a trailing slash. This is causing the mismatch only in the DEV_SSR environment.

During a production build SSR IS able to report on isCurrent and the client and server code indeed match.

We are (have been for a few years) using the gatsby-plugin-remove-trailing-slashes plugin.

Steps to reproduce

1. Clone this reproduction repo
2. yarn install
3. yarn start
4. browse to http://localhost:8000/test (no trailing slash)
5. Hard/Full reload the page at least once. (this triggers SSR)
6. View the following error in the console....
   

If you wish to further test...

1. Implement the gatsby-plugin-remove-trailing-slashes
2. yarn build && yarn serve
3. Browse to http://localhost:9000/test (no trailing slash)
4. Compare Server vs Client for that Gatsby Link and notice that they both match.

Expected result

That with DEV_SSR: true it should NOT error/warning about this as it will not be an issue in build.
It seems that DEV_SSR will need to properly be able to detect location in order for this to work.

Actual result

With DEV_SSR: true it is not properly reporting location during SSR only.

Environment

  System:
    OS: macOS 10.15.7
    CPU: (12) x64 Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
    Shell: 5.7.1 - /bin/zsh
  Binaries:
    Node: 14.15.1 - ~/.nvm/versions/node/v14.15.1/bin/node
    Yarn: 1.22.10 - ~/.nvm/versions/node/v14.15.1/bin/yarn
    npm: 6.14.8 - ~/.nvm/versions/node/v14.15.1/bin/npm
  Languages:
    Python: 2.7.16 - /usr/bin/python
  Browsers:
    Chrome: 89.0.4389.114
    Firefox: 84.0
    Safari: 14.0.3
  npmPackages:
    gatsby: ^3.1.2 => 3.2.1 
    gatsby-plugin-gatsby-cloud: ^2.1.0 => 2.2.0 
    gatsby-plugin-image: ^1.1.2 => 1.2.1 
    gatsby-plugin-manifest: ^3.1.0 => 3.2.0 
    gatsby-plugin-offline: ^4.1.0 => 4.2.0 
    gatsby-plugin-react-helmet: ^4.1.0 => 4.2.0 
    gatsby-plugin-sharp: ^3.1.2 => 3.2.1 
    gatsby-source-filesystem: ^3.1.0 => 3.2.0 
    gatsby-transformer-sharp: ^3.1.0 => 3.2.0 
  npmGlobalPackages:
    gatsby-cli: 3.2.0

[LekoArts]

Hi! Please report your issue in the umbrella discussion as this is the single place for bug reports/feature requests/discussion about that flag/experiment: #28138
Thanks!