internal_link: Reject unknown is operand.

Previously, `is` elements with operands other than `mentioned` are
ignored when parsing internal links. When the remaining elements
combined are recognized, some non-null narrows can be returned.

Now `is` elements are never ignored, to avoid such false positives.

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

lib/model/internal_link.dart

@@ -184,7 +184,11 @@ Narrow? _interpretNarrowSegments(List<String> segments, PerAccountStore store) {
 
       case _NarrowOperator.is_:
         if (isMentionedElement != null) return null;
-        if (operand == 'mentioned') isMentionedElement = ApiNarrowIsMentioned();
+        if (operand == 'mentioned') {
+          isMentionedElement = ApiNarrowIsMentioned();
+        } else {
+          return null;
+        }
 
       case _NarrowOperator.near: // TODO(#82): support for near
       case _NarrowOperator.with_: // TODO(#683): support for with

test/model/internal_link_test.dart

@@ -239,6 +239,7 @@ void main() {
       final testCases = [
         ('/#narrow/stream/name/topic/',           null), // missing operand
         ('/#narrow/stream/name/unknown/operand/', null), // unknown operator
+        ('/#narrow/is/starred',                   null), // unknown `is` operand
       ];
       testExpectedNarrows(testCases, streams: streams);
     });