message [nfc]: Pull out orig/new values from move as local variables

This lets the Dart compiler see that these are definitely constant
through the function (which it can't assume when they're getters on
an object).  That in turn lets it narrow the types based on our
null checks, so that we can leave out `!` operators later.

That's small now, but will be more useful as we add more logic to
this function -- if we had to scatter `!` all around that logic,
we'd have to wonder whether some of them were unjustified assumptions
and might throw at runtime.

Also rename one local to "newStreamId", matching the property it
mirrors.  We'll rename both the variable and the property together,
as part of a sweep coming up soon (#631).

Author: gnprice

lib/model/message.dart

@@ -148,11 +148,16 @@ class MessageStoreImpl with MessageStore {
     // The interaction between the fields of these events are a bit tricky.
     // For reference, see: https://zulip.com/api/get-events#update_message
 
-    if (event.origTopic == null) {
+    final origStreamId = event.origStreamId;
+    final newStreamId = event.newStreamId; // null if topic-only move
+    final origTopic = event.origTopic;
+    final newTopic = event.newTopic;
+
+    if (origTopic == null) {
       // There was no move.
       assert(() {
-        if (event.newStreamId != null && event.origStreamId != null
-            && event.newStreamId != event.origStreamId) {
+        if (newStreamId != null && origStreamId != null
+            && newStreamId != origStreamId) {
           // This should be impossible; `orig_subject` (aka origTopic) is
           // documented to be present when either the stream or topic changed.
           debugLog('Malformed UpdateMessageEvent: stream move but no origTopic'); // TODO(log)
@@ -162,22 +167,19 @@ class MessageStoreImpl with MessageStore {
       return;
     }
 
-    if (event.newTopic == null) {
+    if (newTopic == null) {
       // The `subject` field (aka newTopic) is documented to be present on moves.
       assert(debugLog('Malformed UpdateMessageEvent: move but no newTopic')); // TODO(log)
       return;
     }
-    if (event.origStreamId == null) {
+    if (origStreamId == null) {
       // The `stream_id` field (aka origStreamId) is documented to be present on moves.
       assert(debugLog('Malformed UpdateMessageEvent: move but no origStreamId')); // TODO(log)
       return;
     }
 
-    final newTopic = event.newTopic!;
-    final newChannelId = event.newStreamId; // null if topic-only move
-
-    if (newChannelId == null
-        && MessageEditState.topicMoveWasResolveOrUnresolve(event.origTopic!, newTopic)) {
+    if (newStreamId == null
+        && MessageEditState.topicMoveWasResolveOrUnresolve(origTopic, newTopic)) {
       // The topic was only resolved/unresolved.
       // No change to the messages' editState.
       return;