Clean up AstProvider hierarchy

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';
+import 'package:analyzer/src/dart/analysis/driver.dart';
 
 /**
  * Provider for resolved and unresolved [CompilationUnit]s that contain, or
  * [AstNode]s that declare [Element]s.
  */
 abstract class AstProvider {
   /**
+   * Return the driver that is used to provide ASTs.
+   */
+  AnalysisDriver get driver;
+
+  /**
    * Completes with the [SimpleIdentifier] that declares the [element]. The
    * enclosing unit is only parsed, but not resolved. Completes with `null` if
    * the [element] is synthetic, or the file where it is declared cannot be
    * parsed, etc.
    */
   Future<SimpleIdentifier> getParsedNameForElement(Element element);
 
   /**
    * Completes with the parsed [CompilationUnit] that contains the [element].
    */
   Future<CompilationUnit> getParsedUnitForElement(Element element);
 
   /**
    * Completes with the [SimpleIdentifier] that declares the [element]. The
    * enclosing unit is fully resolved. Completes with `null` if the [element]
    * is synthetic, or the file where it is declared cannot be parsed and
    * resolved, etc.
    */
   Future<SimpleIdentifier> getResolvedNameForElement(Element element);
 
   /**
    * Completes with the resolved [CompilationUnit] that contains the [element].
    */
   Future<CompilationUnit> getResolvedUnitForElement(Element element);
 }