narrow: Implement toString on {Stream,Topic,Dm}Narrow classes

This will be helpful for test-failure output, particularly with
DmNarrows as we start using those as Map keys, soon.

Of our current Narrow subclasses, this just leaves AllMessagesNarrow
without a custom `toString` implementation. That's OK, because the
default `toString` already says "AllMessagesNarrow", which is all we
need to know about the narrow.

Author: chrisbobbe

lib/model/narrow.dart

@@ -70,6 +70,9 @@ class StreamNarrow extends Narrow {
   @override
   ApiNarrow apiEncode() => [ApiNarrowStream(streamId)];
 
+  @override
+  String toString() => 'StreamNarrow(streamId: $streamId)';
+
   @override
   bool operator ==(Object other) {
     if (other is! StreamNarrow) return false;
@@ -102,6 +105,9 @@ class TopicNarrow extends Narrow implements SendableNarrow {
   @override
   StreamDestination get destination => StreamDestination(streamId, topic);
 
+  @override
+  String toString() => 'TopicNarrow(streamId: $streamId, topic: $topic)';
+
   @override
   bool operator ==(Object other) {
     if (other is! TopicNarrow) return false;
@@ -207,6 +213,9 @@ class DmNarrow extends Narrow implements SendableNarrow {
   @override
   ApiNarrow apiEncode() => [ApiNarrowDm(allRecipientIds)];
 
+  @override
+  String toString() => 'DmNarrow(allRecipientIds: $allRecipientIds)';
+
   @override
   bool operator ==(Object other) {
     if (other is! DmNarrow) return false;