Use null-aware operators to clean up the code

pkg/analyzer/lib/src/context/source.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.source;
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/physical_file_system.dart';
 import 'package:analyzer/source/package_map_resolver.dart';
 import 'package:analyzer/src/generated/engine.dart';
@@ skipping 35 matching lines @@
    * The predicate to determine is [Source] is local.
    */
   LocalSourcePredicate _localSourcePredicate = LocalSourcePredicate.NOT_SDK;
 
   /**
    * Initialize a newly created source factory with the given absolute URI
    * [resolvers] and optional [packages] resolution helper.
    */
   SourceFactoryImpl(this.resolvers,
       [this._packages, ResourceProvider resourceProvider])
-      : _resourceProvider = resourceProvider != null
-            ? resourceProvider
-            : PhysicalResourceProvider.INSTANCE;
+      : _resourceProvider =
+            resourceProvider ?? PhysicalResourceProvider.INSTANCE;
 
   /**
    * Return the [DartSdk] associated with this [SourceFactory], or `null` if
    * there is no such SDK.
    *
    * @return the [DartSdk] associated with this [SourceFactory], or `null` if
    *         there is no such SDK
    */
   @override
   DartSdk get dartSdk {
@@ skipping 29 matching lines @@
             _resourceProvider.getFolder(uri.toFilePath())
           ];
         }
       });
       return packageMap;
     }
 
     // Default to the PackageMapUriResolver.
     PackageMapUriResolver resolver = resolvers
         .firstWhere((r) => r is PackageMapUriResolver, orElse: () => null);
-    return resolver != null ? resolver.packageMap : null;
+    return resolver?.packageMap;
   }
 
   /**
    * Return a source factory that will resolve URI's in the same way that this
    * source factory does.
    */
   @override
   SourceFactory clone() {
     SourceFactory factory =
         new SourceFactory(resolvers, _packages, _resourceProvider);
@@ skipping 195 matching lines @@
     for (UriResolver resolver in resolvers) {
       Source result = resolver.resolveAbsolute(containedUri, actualUri);
       if (result != null) {
         return result;
       }
     }
 
     return null;
   }
 }