Accessors are only available when targeting ECMAScript 5 and higher

From .src/tsconfig.es5.json:

// This file is used by ngc to create the files in the build directory
{
"compilerOptions": {
"declaration": true,
"module": "es2015",
**"target": "es5",**
"baseUrl": ".",
.
.
.

./tsconfig.json doesn't contain the "target" property.

The solution is to add it.

Robert

Compare src/tsconfig.es5.json with tsconfig.json, starting with the compilerOptions target setting described in the issue. Add the missing target to tsconfig.json and verify that the generated Angular library configuration includes the intended ECMAScript target.

Written by the indexing model from the issue text.