refactor(@angular-devkit/schematics): add explicit return types to fu… #996

Job	Run time
list	27s
workflow (modules-builder-tests)	1m 39s
workflow (build-cli)	1m 23s
workflow (devkit-core-tests)	1m 9s
workflow (devkit-core-tests-rerun)	1m 12s
workflow (build-unit-tests)	2m 5s
	7m 55s

Provide feedback