function rxjsAlias(supportES2015) {
  try {
    const rxjsPathMappingImport = supportES2015 ? 'rxjs/_esm2015/path-mapping' : 'rxjs/_esm5/path-mapping';
    const rxPaths = require(rxjsPathMappingImport);
    return rxPaths(helpers.root('node_modules'));
  } catch (e) {
    return {};
  }
}
Read the tsconfig to determine if we should prefer ES2015 modules.
Load rxjs path aliases.
https://github.com/ReactiveX/rxjs/blob/master/doc/lettable-operators.md#build-and-treeshaking
@param supportES2015 Set to true when the output of typescript is >= ES6