Clean up AstProvider hierarchy

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/dart/element/ast_provider.dart';
-import 'package:analyzer/src/generated/engine.dart';
-
-/**
- * [AstProvider] implementation for [AnalysisContext].
- */
-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);
-  }
-}