enable lint prefer_conditional_assignment (#12694)

analysis_options.yaml

@@ -111,7 +111,7 @@ linter:
     - prefer_adjacent_string_concatenation
     # - prefer_asserts_in_initializer_lists # not yet tested
     - prefer_collection_literals
-    # - prefer_conditional_assignment # not yet tested
+    - prefer_conditional_assignment
     - prefer_const_constructors
     # - prefer_const_constructors_in_immutables # not yet tested
     # - prefer_constructors_over_static_methods # not yet tested

analysis_options_repo.yaml

@@ -105,7 +105,7 @@ linter:
     - prefer_adjacent_string_concatenation
     # - prefer_asserts_in_initializer_lists # not yet tested
     - prefer_collection_literals
-    # - prefer_conditional_assignment # not yet tested
+    - prefer_conditional_assignment
     - prefer_const_constructors
     # - prefer_const_constructors_in_immutables # not yet tested
     # - prefer_constructors_over_static_methods # not yet tested

packages/flutter/lib/src/widgets/navigator.dart

@@ -798,8 +798,7 @@ class NavigatorState extends State<Navigator> with TickerProviderStateMixin {
       Route<dynamic> route;
       if (initialRouteName != Navigator.defaultRouteName)
         route = _routeNamed(initialRouteName, allowNull: true);
-      if (route == null)
-        route = _routeNamed(Navigator.defaultRouteName);
+      route ??= _routeNamed(Navigator.defaultRouteName);
       push(route);
     }
     for (Route<dynamic> route in _history)

packages/flutter_tools/lib/src/base/terminal.dart

@@ -85,8 +85,7 @@ class AnsiTerminal {
   ///
   /// Useful when the console is in [singleCharMode].
   Stream<String> get onCharInput {
-    if (_broadcastStdInString == null)
-      _broadcastStdInString = io.stdin.transform(ASCII.decoder).asBroadcastStream();
+    _broadcastStdInString ??= io.stdin.transform(ASCII.decoder).asBroadcastStream();
     return _broadcastStdInString;
   }
 

packages/flutter_tools/lib/src/vmservice.dart

@@ -995,11 +995,9 @@ class Isolate extends ServiceObjectOwner {
   }
 
   void _updateHeaps(Map<String, dynamic> map, bool mapIsRef) {
-    if (_newSpace == null)
-      _newSpace = new HeapSpace._empty(this);
+    _newSpace ??= new HeapSpace._empty(this);
     _newSpace._update(map['new'], mapIsRef);
-    if (_oldSpace == null)
-      _oldSpace = new HeapSpace._empty(this);
+    _oldSpace ??= new HeapSpace._empty(this);
     _oldSpace._update(map['old'], mapIsRef);
   }