Use null-aware operators to clean up the code

pkg/analyzer/lib/src/generated/constant.dart

 // Copyright (c) 2014, 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 analyzer.src.generated.constant;
 
 import 'package:analyzer/context/declared_variables.dart';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/src/dart/constant/evaluation.dart';
 import 'package:analyzer/src/dart/constant/value.dart';
@@ skipping 102 matching lines @@
    * The type system primitives.
    */
   final TypeSystem _typeSystem;
 
   /**
    * Initialize a newly created evaluator to evaluate expressions in the given
    * [source]. The [typeProvider] is the type provider used to access known
    * types.
    */
   ConstantEvaluator(this._source, this._typeProvider, {TypeSystem typeSystem})
-      : _typeSystem = typeSystem != null ? typeSystem : new TypeSystemImpl();
+      : _typeSystem = typeSystem ?? new TypeSystemImpl();
 
   EvaluationResult evaluate(Expression expression) {
     RecordingErrorListener errorListener = new RecordingErrorListener();
     ErrorReporter errorReporter = new ErrorReporter(errorListener, _source);
     DartObjectImpl result = expression.accept(new ConstantVisitor(
         new ConstantEvaluationEngine(_typeProvider, new DeclaredVariables(),
             typeSystem: _typeSystem),
         errorReporter));
     if (result != null) {
       return EvaluationResult.forValue(result);
     }
     return EvaluationResult.forErrors(errorListener.errors);
   }
 }