Update 10.02.2019
This was a problem of the @angular/cli. Updating the version to >= 8.3.22 should fix the issue: see this comment in #16515
ORIGINAL
Basically the build process is running out of memory: see related angular-cli issues #15493, #16515
The recommended remedy is to:   
- update node to the latest version e.g. 12.14.0
- increase the memory for the build process: 
- in your package.jsonchange the"build"script to:node --max_old_space_size=4096 node_modules/@angular/cli/bin/ng build
- in this case the memory is increased to 4GB
 depending on the size of your project you may need more
 
As a quick workaround it could also work to use older versions of angular/cli and build-angular: 
"devDependencies": {
    "@angular-devkit/build-angular": "0.803.20",
    "@angular/cli": "8.3.20",
Another workaround is to disable differential-loading (i.e. skip generation of ES5 bundles), if you want this. Then the build-process will need less memory and may work.