Do not report unresolved names as comflicting with the SDK (issue 27544)

pkg/analyzer/lib/src/generated/error_verifier.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 analyzer.src.generated.error_verifier;
 
 import 'dart:collection';
 import "dart:math" as math;
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 2337 matching lines @@
       String name = element.displayName;
       List<Element> conflictingMembers = element.conflictingElements;
       int count = conflictingMembers.length;
       List<String> libraryNames = new List<String>(count);
       for (int i = 0; i < count; i++) {
         libraryNames[i] = _getLibraryName(conflictingMembers[i]);
       }
       libraryNames.sort();
       _errorReporter.reportErrorForNode(StaticWarningCode.AMBIGUOUS_IMPORT,
           node, [name, StringUtilities.printListOfQuotedNames(libraryNames)]);
-    } else {
+    } else if (element != null) {
       List<Element> sdkElements =
           node.getProperty(LibraryImportScope.conflictingSdkElements);
       if (sdkElements != null) {
         _errorReporter.reportErrorForNode(
             StaticWarningCode.CONFLICTING_DART_IMPORT, node, [
           element.displayName,
           _getLibraryName(sdkElements[0]),
           _getLibraryName(element)
         ]);
       }
@@ skipping 3996 matching lines @@
       return null;
     }
     ClassElement classElement =
         member.getAncestor((element) => element is ClassElement);
     if (classElement == null) {
       return null;
     }
     String name = member.name;
     ClassElement superclass = classElement.supertype?.element;
     while (superclass != null) {
-      ExecutableElement member = superclass.getMethod(name) ?? superclass.getGetter(name) ?? superclass.getSetter(name);
+      ExecutableElement member = superclass.getMethod(name) ??
+          superclass.getGetter(name) ??
+          superclass.getSetter(name);
       if (member != null) {
         return member;
       }
       superclass = superclass.supertype?.element;
     }
     return null;
   }
 
   /**
    * Return the type of the first and only parameter of the given [setter].
@@ skipping 509 matching lines @@
 class _InvocationCollector extends RecursiveAstVisitor {
   final List<String> superCalls = <String>[];
 
   @override
   visitMethodInvocation(MethodInvocation node) {
     if (node.target is SuperExpression) {
       superCalls.add(node.methodName.name);
     }
   }
 }