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

pkg/analyzer/lib/src/dart/analysis/ast_provider_context.dart

+// Copyright (c) 2017, 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.
+
+import 'dart:async';
+
+import 'package:analyzer/dart/ast/ast.dart';
+import 'package:analyzer/dart/element/element.dart';
+import 'package:analyzer/src/dart/analysis/ast_provider_driver.dart';
+import 'package:analyzer/src/generated/engine.dart';
+
+class AstProviderForContext extends AbstractAstProvider {
+  final AnalysisContext context;
+
+  AstProviderForContext(this.context);
+
+  @override
+  Future<CompilationUnit> getParsedUnitForElement(Element element) async {
+    return context.parseCompilationUnit(element.source);
+  }
+
+  @override
+  Future<CompilationUnit> getResolvedUnitForElement(Element element) async {
+    return context.getResolvedCompilationUnit2(
+        element.source, element.librarySource);
+  }
+}