Clean up AstProvider hierarchy

pkg/analyzer/lib/src/dart/element/ast_provider.dart

Index: pkg/analyzer/lib/src/dart/element/ast_provider.dart
diff --git a/pkg/analyzer/lib/src/dart/element/ast_provider.dart b/pkg/analyzer/lib/src/dart/element/ast_provider.dart
index 312a057189f2b1ef051f89c154cc4bccdd359670..ba94d535b4f8eb254e2b9fdabc67d31527c2ba34 100644
--- a/pkg/analyzer/lib/src/dart/element/ast_provider.dart
+++ b/pkg/analyzer/lib/src/dart/element/ast_provider.dart
@@ -6,6 +6,7 @@ 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
@@ -13,6 +14,11 @@ import 'package:analyzer/dart/element/element.dart';
  */
 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