use conditional assignment (#9252)

examples/flutter_gallery/lib/gallery/syntax_highlighter.dart

@@ -60,9 +60,7 @@ abstract class SyntaxHighlighter { // ignore: one_member_abstracts
 class DartSyntaxHighlighter extends SyntaxHighlighter {
   DartSyntaxHighlighter([this._style]) {
     _spans = <_HighlightSpan>[];
-
-    if (_style == null)
-      _style = SyntaxHighlighterStyle.darkThemeStyle();
+    _style ??= SyntaxHighlighterStyle.darkThemeStyle();
   }
 
   SyntaxHighlighterStyle _style;

examples/layers/widgets/spinning_mixed.dart

@@ -78,8 +78,7 @@ Duration timeBase;
 RenderTransform transformBox;
 
 void rotate(Duration timeStamp) {
-  if (timeBase == null)
-    timeBase = timeStamp;
+  timeBase ??= timeStamp;
   final double delta = (timeStamp - timeBase).inMicroseconds.toDouble() / Duration.MICROSECONDS_PER_SECOND; // radians
 
   transformBox.setIdentity();

packages/flutter/lib/src/painting/box_painter.dart

@@ -577,10 +577,8 @@ class BoxShadow {
   static List<BoxShadow> lerpList(List<BoxShadow> a, List<BoxShadow> b, double t) {
     if (a == null && b == null)
       return null;
-    if (a == null)
-      a = <BoxShadow>[];
-    if (b == null)
-      b = <BoxShadow>[];
+    a ??= <BoxShadow>[];
+    b ??= <BoxShadow>[];
     final List<BoxShadow> result = <BoxShadow>[];
     final int commonLength = math.min(a.length, b.length);
     for (int i = 0; i < commonLength; ++i)

packages/flutter/lib/src/rendering/box.dart

@@ -1482,8 +1482,7 @@ abstract class RenderBox extends RenderObject {
   @mustCallSuper
   double getDistanceToActualBaseline(TextBaseline baseline) {
     assert(_debugDoingBaseline);
-    if (_cachedBaselines == null)
-      _cachedBaselines = <TextBaseline, double>{};
+    _cachedBaselines ??= <TextBaseline, double>{};
     _cachedBaselines.putIfAbsent(baseline, () => computeDistanceToActualBaseline(baseline));
     return _cachedBaselines[baseline];
   }

packages/flutter/lib/src/rendering/layer.dart

@@ -203,8 +203,7 @@ class ContainerLayer extends Layer {
     if (_lastChild != null)
       _lastChild._nextSibling = child;
     _lastChild = child;
-    if (_firstChild == null)
-      _firstChild = child;
+    _firstChild ??= child;
   }
 
   void _remove(Layer child) {

packages/flutter/lib/src/rendering/object.dart

@@ -2697,8 +2697,7 @@ abstract class ContainerRenderObjectMixin<ChildType extends RenderObject, Parent
         _firstChildParentData.previousSibling = child;
       }
       _firstChild = child;
-      if (_lastChild == null)
-        _lastChild = child;
+      _lastChild ??= child;
     } else {
       assert(_firstChild != null);
       assert(_lastChild != null);

packages/flutter/lib/src/scheduler/ticker.dart

@@ -199,8 +199,7 @@ class Ticker {
     assert(scheduled);
     _animationId = null;
 
-    if (_startTime == null)
-      _startTime = timeStamp;
+    _startTime ??= timeStamp;
 
     _onTick(timeStamp - _startTime);
 

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

@@ -132,8 +132,7 @@ class FocusScopeNode extends Object with TreeDiagnosticsMixin {
     if (_firstChild != null)
       _firstChild._previousSibling = child;
     _firstChild = child;
-    if (_lastChild == null)
-      _lastChild = child;
+    _lastChild ??= child;
     child._updateManager(_manager);
   }
 

packages/flutter/test/gestures/gesture_binding_test.dart

@@ -24,8 +24,7 @@ class TestGestureFlutterBinding extends BindingBase with GestureBinding {
 TestGestureFlutterBinding _binding = new TestGestureFlutterBinding();
 
 void ensureTestGestureBinding() {
-  if (_binding == null)
-    _binding = new TestGestureFlutterBinding();
+  _binding ??= new TestGestureFlutterBinding();
   assert(GestureBinding.instance != null);
 }
 

packages/flutter_tools/lib/src/android/android_device.dart

@@ -430,12 +430,7 @@ class AndroidDevice extends Device {
   }
 
   @override
-  DevicePortForwarder get portForwarder {
-    if (_portForwarder == null)
-      _portForwarder = new _AndroidDevicePortForwarder(this);
-
-    return _portForwarder;
-  }
+  DevicePortForwarder get portForwarder => _portForwarder ??= new _AndroidDevicePortForwarder(this);
 
   static final RegExp _timeRegExp = new RegExp(r'^\d{2}-\d{2} \d{2}:\d{2}:\d{2}\.\d{3}', multiLine: true);
 

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

@@ -116,8 +116,7 @@ class _PosixUtils extends OperatingSystemUtils {
               .trim()} ${results[2].stdout.trim()}";
         }
       }
-      if (_name == null)
-        _name = super.name;
+      _name ??= super.name;
     }
     return _name;
   }

packages/flutter_tools/lib/src/cache.dart

@@ -86,12 +86,7 @@ class Cache {
 
   static String _dartSdkVersion;
 
-  static String get dartSdkVersion {
-    if (_dartSdkVersion == null) {
-      _dartSdkVersion = platform.version;
-    }
-    return _dartSdkVersion;
-  }
+  static String get dartSdkVersion => _dartSdkVersion ??= platform.version;
 
   static String _engineRevision;
 

packages/flutter_tools/lib/src/commands/daemon.dart

@@ -724,8 +724,7 @@ class AppInstance {
   }
 
   dynamic _runInZone(AppDomain domain, dynamic method()) {
-    if (_logger == null)
-      _logger = new _AppRunLogger(domain, this, logToStdout: logToStdout);
+    _logger ??= new _AppRunLogger(domain, this, logToStdout: logToStdout);
 
     final AppContext appContext = new AppContext();
     appContext.setVariable(Logger, _logger);

packages/flutter_tools/lib/src/dart/pub.dart

@@ -34,8 +34,7 @@ Future<Null> pubGet({
   bool offline: false,
   bool checkLastModified: true
 }) async {
-  if (directory == null)
-    directory = fs.currentDirectory.path;
+  directory ??= fs.currentDirectory.path;
 
   final File pubSpecYaml = fs.file(fs.path.join(directory, 'pubspec.yaml'));
   final File dotPackages = fs.file(fs.path.join(directory, '.packages'));

packages/flutter_tools/lib/src/device.dart

@@ -97,8 +97,7 @@ abstract class PollingDeviceDiscovery extends DeviceDiscovery {
 
   void startPolling() {
     if (_timer == null) {
-      if (_items == null)
-        _items = new ItemListNotifier<Device>();
+      _items ??= new ItemListNotifier<Device>();
       _timer = new Timer.periodic(_pollingDuration, (Timer timer) {
         _items.updateWithNewList(pollingGetDevices());
       });
@@ -112,20 +111,17 @@ abstract class PollingDeviceDiscovery extends DeviceDiscovery {
 
   @override
   List<Device> get devices {
-    if (_items == null)
-      _items = new ItemListNotifier<Device>.from(pollingGetDevices());
+    _items ??= new ItemListNotifier<Device>.from(pollingGetDevices());
     return _items.items;
   }
 
   Stream<Device> get onAdded {
-    if (_items == null)
-      _items = new ItemListNotifier<Device>();
+    _items ??= new ItemListNotifier<Device>();
     return _items.onAdded;
   }
 
   Stream<Device> get onRemoved {
-    if (_items == null)
-      _items = new ItemListNotifier<Device>();
+    _items ??= new ItemListNotifier<Device>();
     return _items.onRemoved;
   }
 

packages/flutter_tools/lib/src/ios/devices.dart

@@ -357,12 +357,7 @@ class IOSDevice extends Device {
   }
 
   @override
-  DevicePortForwarder get portForwarder {
-    if (_portForwarder == null)
-      _portForwarder = new _IOSDevicePortForwarder(this);
-
-    return _portForwarder;
-  }
+  DevicePortForwarder get portForwarder => _portForwarder ??= new _IOSDevicePortForwarder(this);
 
   @override
   void clearLogs() {

packages/flutter_tools/lib/src/ios/simulators.dart

@@ -565,12 +565,7 @@ class IOSSimulator extends Device {
   }
 
   @override
-  DevicePortForwarder get portForwarder {
-    if (_portForwarder == null)
-      _portForwarder = new _IOSSimulatorDevicePortForwarder(this);
-
-    return _portForwarder;
-  }
+  DevicePortForwarder get portForwarder => _portForwarder ??= new _IOSSimulatorDevicePortForwarder(this);
 
   @override
   void clearLogs() {

packages/flutter_tools/lib/src/resident_runner.dart

@@ -406,8 +406,7 @@ class OperationResult {
 /// Given the value of the --target option, return the path of the Dart file
 /// where the app's main function should be.
 String findMainDartFile([String target]) {
-  if (target == null)
-    target = '';
+  target ??= '';
   final String targetPath = fs.path.absolute(target);
   if (fs.isDirectorySync(targetPath))
     return fs.path.join(targetPath, 'lib', 'main.dart');

packages/flutter_tools/lib/src/runner/flutter_command_runner.dart

@@ -286,8 +286,7 @@ class FlutterCommandRunner extends CommandRunner<Null> {
         engineSourcePath = null;
       }
 
-      if (engineSourcePath == null)
-        engineSourcePath = _tryEnginePath(fs.path.join(Cache.flutterRoot, '../engine/src'));
+      engineSourcePath ??= _tryEnginePath(fs.path.join(Cache.flutterRoot, '../engine/src'));
 
       if (engineSourcePath == null) {
         printError('Unable to detect local Flutter engine build directory.\n'
@@ -327,8 +326,7 @@ class FlutterCommandRunner extends CommandRunner<Null> {
   }
 
   static void initFlutterRoot() {
-    if (Cache.flutterRoot == null)
-      Cache.flutterRoot = _defaultFlutterRoot;
+    Cache.flutterRoot ??= _defaultFlutterRoot;
   }
 
   /// Get all pub packages in the Flutter repo.

packages/flutter_tools/lib/src/vmservice.dart

@@ -266,11 +266,9 @@ abstract class ServiceObject {
         serviceObject = new Isolate._empty(owner.vm);
       break;
     }
-    if (serviceObject == null) {
     // If we don't have a model object for this service object type, as a
     // fallback return a ServiceMap object.
-      serviceObject = new ServiceMap._empty(owner);
-    }
+    serviceObject ??= new ServiceMap._empty(owner);
     // We have now constructed an emtpy service object, call update to
     // populate it.
     serviceObject.update(map);

packages/flutter_tools/lib/src/vmservice_record_replay.dart

@@ -43,19 +43,12 @@ class RecordingVMServiceChannel extends DelegatingStreamChannel<String> {
 
   @override
   Stream<String> get stream {
-    if (_streamRecorder == null) {
-      _streamRecorder = new _RecordingStream(super.stream, _messages);
-    }
+    _streamRecorder ??= new _RecordingStream(super.stream, _messages);
     return _streamRecorder.stream;
   }
 
   @override
-  StreamSink<String> get sink {
-    if (_sinkRecorder == null) {
-      _sinkRecorder = new _RecordingSink(super.sink, _messages);
-    }
-    return _sinkRecorder;
-  }
+  StreamSink<String> get sink => _sinkRecorder ??= new _RecordingSink(super.sink, _messages);
 }
 
 /// Base class for request and response JSON-rpc messages.
@@ -250,11 +243,7 @@ class ReplayVMServiceChannel extends StreamChannelMixin<String> {
   }
 
   @override
-  StreamSink<String> get sink {
-    if (_replaySink == null)
-      _replaySink = new _ReplaySink(this);
-    return _replaySink;
-  }
+  StreamSink<String> get sink => _replaySink ??= new _ReplaySink(this);
 
   @override
   Stream<String> get stream => _controller.stream;