Use null aware operators to clean up code

pkg/analysis_server/lib/src/operation/operation_analysis.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 operation.analysis;
 
 import 'package:analysis_server/src/analysis_server.dart';
 import 'package:analysis_server/src/computer/computer_highlights.dart';
 import 'package:analysis_server/src/computer/computer_highlights2.dart';
 import 'package:analysis_server/src/computer/computer_outline.dart';
@@ skipping 71 matching lines @@
     AnalysisContext context,
     CompilationUnit parsedDartUnit,
     CompilationUnit resolvedDartUnit,
     List<AnalysisError> errors) {
   // If the file belongs to any analysis root, check whether we're in it now.
   AnalysisContext containingContext = server.getContainingContext(file);
   if (containingContext != null && context != containingContext) {
     return;
   }
   // Dart
-  CompilationUnit dartUnit =
-      resolvedDartUnit != null ? resolvedDartUnit : parsedDartUnit;
+  CompilationUnit dartUnit = resolvedDartUnit ?? parsedDartUnit;
   if (resolvedDartUnit != null) {
     if (server.hasAnalysisSubscription(
         protocol.AnalysisService.HIGHLIGHTS, file)) {
       server.scheduleOperation(
           new _DartHighlightsOperation(context, file, resolvedDartUnit));
     }
     if (server.hasAnalysisSubscription(
         protocol.AnalysisService.NAVIGATION, file)) {
       server.scheduleOperation(new NavigationOperation(context, source));
     }
@@ skipping 437 matching lines @@
 abstract class _SingleFileOperation extends SourceSensitiveOperation {
   final String file;
 
   _SingleFileOperation(AnalysisContext context, this.file) : super(context);
 
   @override
   bool shouldBeDiscardedOnSourceChange(Source source) {
     return source.fullName == file;
   }
 }