Use null-aware operators to clean up the code

pkg/analyzer/lib/src/task/strong/checker.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.
 
 // TODO(jmesserly): this was ported from package:dev_compiler, and needs to be
 // refactored to fit into analyzer.
 library analyzer.src.task.strong.checker;
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 307 matching lines @@
         var staticInfo =
             new InvalidParameterDeclaration(rules, node, fieldType);
         _recordMessage(staticInfo);
       }
     }
     node.visitChildren(this);
   }
 
   @override
   void visitForEachStatement(ForEachStatement node) {
-    var loopVariable = node.identifier != null
-        ? node.identifier
-        : node.loopVariable?.identifier;
+    var loopVariable = node.identifier ?? node.loopVariable?.identifier;
 
     // Safely handle malformed statements.
     if (loopVariable != null) {
       // Find the element type of the sequence.
       var sequenceInterface = node.awaitKeyword != null
           ? typeProvider.streamType
           : typeProvider.iterableType;
       var iterableType = _getStaticType(node.iterable);
       var elementType =
           rules.mostSpecificTypeArgument(iterableType, sequenceInterface);
@@ skipping 786 matching lines @@
       if (node != null) {
         _checkIndividualOverridesFromClass(node, interfaceType, seen, false);
       } else {
         _checkIndividualOverridesFromType(
             type, interfaceType, errorLocation, seen, false);
       }
     }
 
     // Check overrides from its mixins
     for (int i = 0; i < type.mixins.length; i++) {
-      var loc =
-          errorLocation != null ? errorLocation : node.withClause.mixinTypes[i];
+      var loc = errorLocation ?? node.withClause.mixinTypes[i];
       for (var interfaceType in interfaces) {
         // We copy [seen] so we can report separately if more than one mixin or
         // the base class have an invalid override.
         _checkIndividualOverridesFromType(
             type.mixins[i], interfaceType, loc, new Set.from(seen), false);
       }
     }
 
     // Check overrides from its superclasses
     if (includeParents) {
       var parent = type.superclass;
-      if (parent.isObject) return;
-      var loc = errorLocation != null ? errorLocation : node.extendsClause;
+      if (parent.isObject) {
+        return;
+      }
+      var loc = errorLocation ?? node.extendsClause;
       // No need to copy [seen] here because we made copies above when reporting
       // errors on mixins.
       _checkInterfacesOverrides(parent, interfaces, seen,
           visited: visited, includeParents: true, errorLocation: loc);
     }
   }
 
   /// Check overrides from mixin applications themselves. For example, in:
   ///
   ///      A extends B with E, F
@@ skipping 131 matching lines @@
     } while (!current.isObject && !visited.contains(current));
   }
 
   void _recordMessage(StaticInfo info) {
     if (info == null) return;
     var error = info.toAnalysisError();
     if (error.errorCode.errorSeverity == ErrorSeverity.ERROR) _failure = true;
     _reporter.onError(error);
   }
 }