Use assert_instances_of() in Differential

Summary: NOTE: This is not produced by a script so there might be errors. Please review carefully.

Test Plan: Browse around Differential.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran

Differential Revision: https://secure.phabricator.com/D2103

src/docs/contributing/php_coding_standards.diviner

@@ -61,6 +61,7 @@ guidelines, you probably don't need to read this super thoroughly.
   - Prefer class constants over defines.
   - Avoid naked class properties; instead, define accessors.
   - Use exceptions for error conditions.
+  - Use type hints, use `assert_instances_of()` for arrays holding objects.
 
 = Examples =