Suppress follow-on errors when a file is imported with either a prefix or a show clause

pkg/analysis_server/lib/src/services/correction/namespace.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 services.src.correction.namespace;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/dart/resolver/scope.dart';
 
@@ skipping 19 matching lines @@
 /**
  * Returns the export namespace of the given [LibraryElement].
  */
 Map<String, Element> getExportNamespaceForLibrary(LibraryElement library) {
   Namespace namespace =
       new NamespaceBuilder().createExportNamespaceForLibrary(library);
   return namespace.definedNames;
 }
 
 /**
- * Returns the [ImportElement] that is referenced by [prefixNode] with
- * an [PrefixElement], maybe `null`.
+ * Return the [ImportElement] that is referenced by [prefixNode], or `null` if
+ * the node does not reference a prefix or if we cannot determine which import
+ * is being referenced.
  */
 ImportElement getImportElement(SimpleIdentifier prefixNode) {
-  if (prefixNode.parent is ImportDirective) {
-    ImportDirective importDirective = prefixNode.parent;
-    return importDirective.element;
+  AstNode parent = prefixNode.parent;
+  if (parent is ImportDirective) {
+    return parent.element;
   }
   ImportElementInfo info = internal_getImportElementInfo(prefixNode);
   return info?.element;
 }
 
 /**
  * Return the [ImportElement] that declared [prefix] and imports [element].
  *
  * [libraryElement] - the [LibraryElement] where reference is.
  * [prefix] - the import prefix, maybe `null`.
@@ skipping 116 matching lines @@
 }
 
 /**
  * Information about [ImportElement] and place where it is referenced using
  * [PrefixElement].
  */
 class ImportElementInfo {
   ImportElement element;
   int periodEnd;
 }