Elements. Update FindElement2 to work with elements, not fragments.

Related to: https://dart-review.googlesource.com/c/sdk/+/390812

No more `Fragment.name2` uses in `FindElement2`.

Change-Id: Idb5e766f4ced1b63bb2154d6fb1bdf5582011d73
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/390633
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

pkg/analysis_server/test/abstract_single_unit.dart

@@ -9,7 +9,7 @@ import 'package:analyzer/error/error.dart';
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/src/error/codes.g.dart';
 import 'package:analyzer/src/test_utilities/find_element.dart';
-import 'package:analyzer/src/test_utilities/find_element_2.dart';
+import 'package:analyzer/src/test_utilities/find_element2.dart';
 import 'package:analyzer/src/test_utilities/find_node.dart';
 import 'package:test/test.dart';
 

pkg/analysis_server/test/services/search/search_engine_test.dart

@@ -8,7 +8,7 @@ import 'package:analyzer/dart/analysis/results.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/src/test_utilities/find_element.dart';
-import 'package:analyzer/src/test_utilities/find_element_2.dart';
+import 'package:analyzer/src/test_utilities/find_element2.dart';
 import 'package:analyzer/src/test_utilities/find_node.dart';
 import 'package:analyzer/src/test_utilities/package_config_file_builder.dart';
 import 'package:analyzer/src/util/file_paths.dart';

pkg/analyzer/lib/src/test_utilities/find_element.dart

@@ -248,10 +248,9 @@ class FindElement extends _FindElementBase {
 
   PrefixElement prefix(String name) {
     for (var libraryFragment in unitElement.withEnclosing) {
-      for (var import_ in libraryFragment.libraryImports) {
-        var prefix = import_.prefix?.element;
-        if (prefix != null && prefix.name == name) {
-          return prefix;
+      for (var importPrefix in libraryFragment.libraryImportPrefixes) {
+        if (importPrefix.name == name) {
+          return importPrefix;
         }
       }
     }