Stop using Element.computeNode() in refactorings and fixes.

pkg/analysis_server/lib/plugin/edit/fix/fix_dart.dart

Index: pkg/analysis_server/lib/plugin/edit/fix/fix_dart.dart
diff --git a/pkg/analysis_server/lib/plugin/edit/fix/fix_dart.dart b/pkg/analysis_server/lib/plugin/edit/fix/fix_dart.dart
index 7b5d3d5970769cec40f5a947686306b5e8ccc74f..d2159d227429e29a687d30c9dbc4eb858fff0e86 100644
--- a/pkg/analysis_server/lib/plugin/edit/fix/fix_dart.dart
+++ b/pkg/analysis_server/lib/plugin/edit/fix/fix_dart.dart
@@ -13,7 +13,9 @@ import 'package:analysis_server/src/services/correction/namespace.dart'
     show getExportedElement;
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
+import 'package:analyzer/src/dart/analysis/ast_provider_context.dart';
 import 'package:analyzer/src/dart/analysis/top_level_declaration.dart';
+import 'package:analyzer/src/dart/element/ast_provider.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/task/dart.dart' show LIBRARY_ELEMENT4;
@@ -31,6 +33,11 @@ typedef Future<List<TopLevelDeclarationInSource>> GetTopLevelDeclarations(
  */
 abstract class DartFixContext implements FixContext {
   /**
+   * The provider for parsed or resolved ASTs.
+   */
+  AstProvider get astProvider;
+
+  /**
    * The function to get top-level declarations from.
    */
   GetTopLevelDeclarations get getTopLevelDeclarations;
@@ -65,7 +72,10 @@ abstract class DartFixContributor implements FixContributor {
       return Fix.EMPTY_LIST;
     }
     DartFixContext dartContext = new DartFixContextImpl(
-        context, _getTopLevelDeclarations(analysisContext), unit);
+        context,
+        _getTopLevelDeclarations(analysisContext),
+        new AstProviderForContext(analysisContext),
+        unit);
     return internalComputeFixes(dartContext);
   }