Use null-aware operators to clean up the code

pkg/analyzer/lib/src/context/cache.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 analyzer.src.context.cache;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analyzer/src/dart/element/element.dart'
@@ skipping 496 matching lines @@
 //          'setValue $descriptor for $target value=$valueStr $dependedOn=$dependedOn');
 //    }
     _validateStateChange(descriptor, CacheState.VALID);
     TargetedResult thisResult = new TargetedResult(target, descriptor);
     if (_partition != null) {
       _partition.resultStored(thisResult, value);
     }
     ResultData data = getResultData(descriptor);
     _setDependedOnResults(data, thisResult, dependedOn);
     data.state = CacheState.VALID;
-    data.value = value == null ? descriptor.defaultValue : value;
+    data.value = value ?? descriptor.defaultValue;
   }
 
   /**
    * If the result represented by the given [descriptor] is valid, set
    * it to the given [value], keep its dependency, and if [invalidateDependent]
    * invalidate all the dependent result.
    */
   void setValueIncremental(
       ResultDescriptor descriptor, dynamic value, bool invalidateDependent) {
     ResultData data = getResultData(descriptor);
@@ skipping 387 matching lines @@
    * Return the entry associated with the given [target].
    */
   CacheEntry get(AnalysisTarget target) => entryMap[target];
 
   /**
    * Return [Source]s whose full path is equal to the given [path].
    * Maybe empty, but not `null`.
    */
   List<Source> getSourcesWithFullName(String path) {
     List<Source> sources = pathToSource[path];
-    return sources != null ? sources : Source.EMPTY_LIST;
+    return sources ?? Source.EMPTY_LIST;
   }
 
   /**
    * Return `true` if this partition is responsible for the given [target].
    */
   bool isResponsibleFor(AnalysisTarget target);
 
   /**
    * Return an iterator returning all of the map entries mapping targets to
    * cache entries.
@@ skipping 343 matching lines @@
   void resultAccessed(TargetedResult result) {}
 
   @override
   List<TargetedResult> resultStored(TargetedResult newResult, newValue) {
     return TargetedResult.EMPTY_LIST;
   }
 
   @override
   void targetRemoved(AnalysisTarget target) {}
 }