Issue #2227731 by _nod: Replace JSHint with ESLint for JavaScript validation.
Issue #2264241 by _nod: Fix ESLint errors and fix js bugs.
Issue #2273311 by donquixote, xjm: Fixed FieldDefinitionTestBase::getNamespacePath() breaks with PSR-4.
Revert "Issue #2227731 by _nod: Replace JSHint with ESLint for JavaScript validation."

This reverts commit 0b522f36cb211485e73483f621f038f611365578.
Issue #2264205 by _nod: Replace JSHint with ESLint for JavaScript validation.