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

pkg/analyzer/lib/src/dart/element/ast_provider.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';
+
+/**
+ * TODO(scheglov) document

[Brian Wilkerson, 2017/02/02 21:07:01]

Do you want to do this before committing this CL?

[scheglov, 2017/02/02 21:08:03]

Acknowledged.

+ */
+abstract class AstProvider {
+  AstNode findNodeForElement(CompilationUnit unit, Element element);
+
+  Future<T> getParsedNodeForElement<T extends AstNode>(Element element);
+
+  Future<CompilationUnit> getParsedUnitForElement(Element element);
+
+  Future<T> getResolvedNodeForElement<T extends AstNode>(Element element);
+
+  Future<CompilationUnit> getResolvedUnitForElement(Element element);
+}