Use null-aware operators to clean up the code

pkg/analyzer/lib/src/context/context.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.context;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 825 matching lines @@
   TimestampedData<String> getContents(Source source) {
     String contents = _contentCache.getContents(source);
     if (contents != null) {
       return new TimestampedData<String>(
           _contentCache.getModificationStamp(source), contents);
     }
     return source.contents;
   }
 
   @override
-  InternalAnalysisContext getContextFor(Source source) {
-    InternalAnalysisContext context = _cache.getContextFor(source);
-    return context == null ? this : context;
-  }
+  InternalAnalysisContext getContextFor(Source source) =>
+      _cache.getContextFor(source) ?? this;
 
   @override
   Element getElement(ElementLocation location) {
     // TODO(brianwilkerson) This should not be a "get" method.
     try {
       List<String> components = location.components;
       Source source = _computeSourceFromEncoding(components[0]);
       String sourceName = source.shortName;
       if (AnalysisEngine.isDartFileName(sourceName)) {
         ElementImpl element = computeLibraryElement(source) as ElementImpl;
@@ skipping 1335 matching lines @@
     }
     DartSdk sdk = factory.dartSdk;
     if (sdk == null) {
       throw new IllegalArgumentException(
           "The source factory for an SDK analysis context must have a DartUriResolver");
     }
     return new AnalysisCache(
         <CachePartition>[AnalysisEngine.instance.partitionManager.forSdk(sdk)]);
   }
 }