Enable the getter-return, no-dupe-else-if, and no-setter-return ESLint rules

All of these rules can help catch errors during development. Please note that only `getter-return` required a few changes, which was limited to disabling the rule in a couple of spots; please find additional details about these rules at:
 - https://eslint.org/docs/rules/getter-return
 - https://eslint.org/docs/rules/no-dupe-else-if
 - https://eslint.org/docs/rules/no-setter-return

web/interfaces.js

@@ -12,6 +12,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+/* eslint-disable getter-return */
 
 /**
  * @interface