"yarn build" fails to build in Windows 10

What?

yarn build on Windows 10 fails to build (while yarn dev runs smoothly).

How to reproduce the error

Run the following command

yarn build

Detailed Error Message

PS C:\misc\sources\Github\pr\reactjs.org> yarn build                                                                                                                                      
yarn run v1.12.1                                                                                                                                                                          
$ gatsby build                                                                                                                                                                            
success open and validate gatsby-config — 0.005 s                                                                                                                                         
success load plugins — 0.223 s                                                                                                                                                            
success onPreInit — 0.931 s                                                                                                                                                               
success delete html and css files from previous builds — 0.183 s                                                                                                                          
success initialize cache — 0.009 s                                                                                                                                                        
success copy gatsby files — 0.333 s                                                                                                                                                       
success onPreBootstrap — 0.021 s                                                                                                                                                          
success source and transform nodes — 0.663 s                                                                                                                                              
success building schema — 0.384 s                                                                                                                                                         
success createPages — 0.347 s                                                                                                                                                             
success createPagesStatefully — 0.041 s                                                                                                                                                   
success onPreExtractQueries — 0.015 s                                                                                                                                                     
success update schema — 0.171 s                                                                                                                                                           
success extract queries from components — 0.144 s                                                                                                                                         
success run graphql queries — 0.197 s — 69/69 353.97 queries/second                                                                                                                       
success write out page data — 0.031 s                                                                                                                                                     
success write out redirect data — 0.012 s                                                                                                                                                 
success onPostBootstrap — 0.001 s                                                                                                                                                         
                                                                                                                                                                                          
info bootstrap finished - 5.941 s                                                                                                                                                         
                                                                                                                                                                                          
success Building production JavaScript and CSS bundles — 5.190 s                                                                                                                          
                                                                                                                                                                                          
error Building static HTML for pages failed                                                                                                                                               
                                                                                                                                                                                          
See our docs page on debugging HTML builds for help https://goo.gl/yL9lND                                                                                                                 
                                                                                                                                                                                          
   5 |          function __webpack_require__(moduleId) {                                                                                                                                  
   6 |                                                                                                                                                                                    
>  7 |                  // Check if module is in cache                                                                                                                                    
     | ^                                                                                                                                                                                  
   8 |                  if(installedModules[moduleId]) {                                                                                                                                  
   9 |                          return installedModules[moduleId].exports;                                                                                                                
  10 |                  }                                                                                                                                                                 
                                                                                                                                                                                          
                                                                                                                                                                                          
  WebpackError: C:\misc\sources\Github\pr\reactjs.org\public\C:\misc\sources\Github\pr\reactjs.org\examples\16-3-release-blog-post\context-example contains invalid WIN32 path characters.
                                                                                                                                                                                          
  - bootstrap:7 Object.mkdirs                                                                                                                                                             
    lib/webpack/bootstrap:7:1                                                                                                                                                             
                                                                                                                                                                                          
  - universalModuleDefinition:5 Object.mkdirs                                                                                                                                             
    lib/webpack/universalModuleDefinition:5:1                                                                                                                                             
                                                                                                                                                                                          
  - bootstrap:9 pathExists                                                                                                                                                                
    lib/webpack/bootstrap:9:1                                                                                                                                                             
                                                                                                                                                                                          
  - bootstrap:12 fn.apply.then.r                                                                                                                                                          
    lib/webpack/bootstrap:12:1                                                                                                                                                            
                                                                                                                                                                                          
                                                                                                                                                                                          
error Command failed with exit code 1.                                                                                                                                                    
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.                                                                                                      

Error Demo

Following image shows that yarn dev runs without an issue while yarn build fails.

Environment

• Fork: https://github.com/dance2die/reactjs.org
• Yarn version: 1.12.1
• Node version: 10.1.0
• Git version: 2.17.1.windows.2
• OS
  

Start with the yarn build entry point and reproduce the failure during Gatsby's static HTML build on Windows 10. Trace the reported invalid WIN32 path through the Webpack build output and path handling; done means the production build completes successfully on Windows while yarn dev continues to work.