internal_link [nfc]: Factor out constructing fragment in its own method

This will make it easy to use the fragment string in several other
places, such as in the next commits where we need to create a fallback
markdown link for a channel.

Author: sm-sayedi

lib/model/internal_link.dart

@@ -58,6 +58,18 @@ String? decodeHashComponent(String str) {
 // When you want to point the server to a location in a message list, you
 // you do so by passing the `anchor` param.
 Uri narrowLink(PerAccountStore store, Narrow narrow, {int? nearMessageId}) {
+  final fragment = narrowLinkFragment(store, narrow, nearMessageId: nearMessageId);
+  Uri result = store.realmUrl.replace(fragment: fragment);
+  if (result.path.isEmpty) {
+    // Always ensure that there is a '/' right after the hostname.
+    // A generated URL without '/' looks odd,
+    // and if used in a Zulip message does not get automatically linkified.
+    result = result.replace(path: '/');
+  }
+  return result;
+}
+
+String narrowLinkFragment(PerAccountStore store, Narrow narrow, {int? nearMessageId}) {
   // TODO(server-7)
   final apiNarrow = resolveApiNarrowForServer(
     narrow.apiEncode(), store.zulipFeatureLevel);
@@ -101,14 +113,7 @@ Uri narrowLink(PerAccountStore store, Narrow narrow, {int? nearMessageId}) {
     fragment.write('/near/$nearMessageId');
   }
 
-  Uri result = store.realmUrl.replace(fragment: fragment.toString());
-  if (result.path.isEmpty) {
-    // Always ensure that there is a '/' right after the hostname.
-    // A generated URL without '/' looks odd,
-    // and if used in a Zulip message does not get automatically linkified.
-    result = result.replace(path: '/');
-  }
-  return result;
+  return fragment.toString();
 }
 
 /// The result of parsing some URL within a Zulip realm,