implement LabeledStatement and Break in kernel

pkg/compiler/lib/src/resolution/tree_elements.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.resolution.tree_elements;
 
 import '../common.dart';
 import '../constants/expressions.dart';
 import '../dart_types.dart';
 import '../diagnostics/source_span.dart';
@@ skipping 343 matching lines @@
     return this[node];
   }
 
   @override
   ConstructorElement getRedirectingTargetConstructor(
       RedirectingFactoryBody node) {
     return this[node];
   }
 
   void defineTarget(Node node, JumpTarget target) {
-    if (_definedTargets == null) {
-      _definedTargets = new Maplet<Node, JumpTarget>();
-    }
+    _definedTargets ??= new Maplet<Node, JumpTarget>();
     _definedTargets[node] = target;
   }
 
   void undefineTarget(Node node) {
     if (_definedTargets != null) {
       _definedTargets.remove(node);
       if (_definedTargets.isEmpty) {
         _definedTargets = null;
       }
     }
   }
 
   @override
   JumpTarget getTargetDefinition(Node node) {
     return _definedTargets != null ? _definedTargets[node] : null;
   }
 
   void registerTargetOf(GotoStatement node, JumpTarget target) {
-    if (_usedTargets == null) {
-      _usedTargets = new Maplet<GotoStatement, JumpTarget>();
-    }
+    _usedTargets ??= new Maplet<GotoStatement, JumpTarget>();
     _usedTargets[node] = target;
   }
 
   @override
   JumpTarget getTargetOf(GotoStatement node) {
     return _usedTargets != null ? _usedTargets[node] : null;
   }
 
   void defineLabel(Label label, LabelDefinition target) {
-    if (_definedLabels == null) {
-      _definedLabels = new Maplet<Label, LabelDefinition>();
-    }
+    _definedLabels ??= new Maplet<Label, LabelDefinition>();
     _definedLabels[label] = target;
   }
 
   void undefineLabel(Label label) {
     if (_definedLabels != null) {
       _definedLabels.remove(label);
       if (_definedLabels.isEmpty) {
         _definedLabels = null;
       }
     }
@@ skipping 23 matching lines @@
       _nativeData = <Node, dynamic>{};
     }
     _nativeData[node] = nativeData;
   }
 
   @override
   dynamic getNativeData(Node node) {
     return _nativeData != null ? _nativeData[node] : null;
   }
 }