tools/minify-js: Improve type checking.

* Use Optional where needed.
* Add type to variables because otherwise it'll be Any.

tools/minify-js

@@ -21,7 +21,7 @@ prev_deploy = args.prev_deploy
 # JavaScript source files to minify (and what output files to create).
 sys.path.insert(0, os.path.join(os.path.dirname(__file__), '..'))
 import scripts.lib.setup_path_on_import
-from typing import Optional, Set
+from typing import Any, Dict, Optional, Set
 
 os.environ['DJANGO_SETTINGS_MODULE'] = 'zproject.settings'
 from django.conf import settings
@@ -37,7 +37,7 @@ subprocess.check_call(['tools/compile-handlebars-templates'])
 subprocess.check_call(['tools/webpack'])
 
 def get_changed_source_files(other_checkout):
-    # type: (Optional[str]) -> Optional[Set[str]]
+    # type: (str) -> Optional[Set[str]]
     """ Get list of changed static files since other_checkout.
     If git fails to return a reasonable looking list, this returns None,
     in which case it should be assumed no files can be reused from
@@ -78,9 +78,11 @@ def get_changed_source_files(other_checkout):
 
 changed_files = set()  # type: Set[str]
 if prev_deploy:
-    changed_files = get_changed_source_files(prev_deploy)
-    if changed_files is None:
+    changed_files_tmp = get_changed_source_files(prev_deploy)
+    if changed_files_tmp is None:
         prev_deploy = None
+    else:
+        changed_files = changed_files_tmp
 
 # Always use the newly compiled handlebars templates and webpack bundle.
 if prev_deploy:
@@ -102,7 +104,10 @@ MIN_DIR = os.path.join(STATIC_PATH, 'min/')
 MAP_DIR = os.path.join(STATIC_PATH, 'source-map/')
 subprocess.check_call(['mkdir', '-p', MIN_DIR, MAP_DIR])
 
-for js_group, filespec in six.iteritems(JS_SPECS):
+for js_group_filespec_pair in six.iteritems(JS_SPECS):
+    # JS_SPECS is not typed, so forcefully type keys and values being read from JS_SPECS
+    js_group = js_group_filespec_pair[0]  # type: str
+    filespec = js_group_filespec_pair[1]  # type: Dict[str, Any]
     #      JS_SPECS look like        'js/foobar.js'.
     # changed_files look like 'static/js/foobar.js'.
     # So we prepend 'static/' to the JS_SPECS so these match up.