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

pkg/analyzer/lib/src/generated/error_verifier.dart

Index: pkg/analyzer/lib/src/generated/error_verifier.dart
diff --git a/pkg/analyzer/lib/src/generated/error_verifier.dart b/pkg/analyzer/lib/src/generated/error_verifier.dart
index 13964bab2dead6462c894514154993267ee22831..792e5db72d251a5e32a28a775aed779fc66f9b7b 100644
--- a/pkg/analyzer/lib/src/generated/error_verifier.dart
+++ b/pkg/analyzer/lib/src/generated/error_verifier.dart
@@ -2355,7 +2355,7 @@ class ErrorVerifier extends RecursiveAstVisitor<Object> {
       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) {
@@ -6372,7 +6372,9 @@ class ErrorVerifier extends RecursiveAstVisitor<Object> {
     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;
       }