use some null aware ops

lib/src/codegen/js_codegen.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 dev_compiler.src.codegen.js_codegen;
 
 import 'dart:collection' show HashSet, HashMap, SplayTreeSet;
 
 import 'package:analyzer/analyzer.dart' hide ConstantEvaluator;
 import 'package:analyzer/src/generated/ast.dart' hide ConstantEvaluator;
@@ skipping 193 matching lines @@
 
     var dartxImport =
         js.statement("let # = #.dartx;", [_dartxVar, _runtimeLibVar]);
 
     var module = js.call("function(#) { 'use strict'; #; #; }",
         [params, dartxImport, _moduleItems]);
 
     var program = <JS.Statement>[
       js.statement("dart_library.library(#, #, #, #, #)", [
         js.string(jsPath, "'"),
-        jsDefaultValue != null ? jsDefaultValue : new JS.LiteralNull(),
+        jsDefaultValue ?? new JS.LiteralNull(),
         js.commentExpression(
             "Imports", new JS.ArrayInitializer(imports, multiline: true)),
         js.commentExpression("Lazy imports",
             new JS.ArrayInitializer(lazyImports, multiline: true)),
         module
       ])
     ];
 
     return new JS.Program(program);
   }
@@ skipping 1627 matching lines @@
 
     _lazyFields.add(field);
 
     return _statement(body);
   }
 
   JS.Expression _visitInitializer(VariableDeclaration node) {
     var value = _visit(node.initializer);
     // explicitly initialize to null, to avoid getting `undefined`.
     // TODO(jmesserly): do this only for vars that aren't definitely assigned.
-    return value != null ? value : new JS.LiteralNull();
+    return value ?? new JS.LiteralNull();
   }
 
   void _flushLazyFields(List<JS.Statement> body) {
     if (_lazyFields.isEmpty) return;
     body.add(_emitLazyFields(_lazyFields));
     _lazyFields.clear();
   }
 
   JS.Statement _emitLazyFields(List<VariableDeclaration> fields) {
     var methods = [];
@@ skipping 667 matching lines @@
       new JS.Yield(js.call('#.moveNext()', iter)),
       init,
       _visit(node.body),
       new JS.Yield(js.call('#.cancel()', iter))
     ]);
   }
 
   @override
   visitBreakStatement(BreakStatement node) {
     var label = node.label;
-    return new JS.Break(label != null ? label.name : null);
+    return new JS.Break(label?.name);
   }
 
   @override
   visitContinueStatement(ContinueStatement node) {
     var label = node.label;
-    return new JS.Continue(label != null ? label.name : null);
+    return new JS.Continue(label?.name);
   }
 
   @override
   visitTryStatement(TryStatement node) {
     return new JS.Try(_visit(node.body), _visitCatch(node.catchClauses),
         _visit(node.finallyBlock));
   }
 
   _visitCatch(NodeList<CatchClause> clauses) {
     if (clauses == null || clauses.isEmpty) return null;
@@ skipping 412 matching lines @@
 
 /// A special kind of element created by the compiler, signifying a temporary
 /// variable. These objects use instance equality, and should be shared
 /// everywhere in the tree where they are treated as the same variable.
 class TemporaryVariableElement extends LocalVariableElementImpl {
   TemporaryVariableElement.forNode(Identifier name) : super.forNode(name);
 
   int get hashCode => identityHashCode(this);
   bool operator ==(Object other) => identical(this, other);
 }