Update MockSdk to be more consistent with the actual SDK, switch x_Driver tests to it.

pkg/analyzer/test/src/dart/analysis/physical_sdk.dart

Index: pkg/analyzer/test/src/dart/analysis/physical_sdk.dart
diff --git a/pkg/analyzer/test/src/dart/analysis/physical_sdk.dart b/pkg/analyzer/test/src/dart/analysis/physical_sdk.dart
deleted file mode 100644
index 665c80a76726bc48cf3c2f327e121a0ec17c6e19..0000000000000000000000000000000000000000
--- a/pkg/analyzer/test/src/dart/analysis/physical_sdk.dart
+++ /dev/null
@@ -1,17 +0,0 @@
-// Copyright (c) 2016, 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 'package:analyzer/file_system/physical_file_system.dart';
-import 'package:analyzer/src/dart/sdk/sdk.dart';
-import 'package:analyzer/src/generated/engine.dart' show AnalysisOptionsImpl;
-import 'package:analyzer/src/generated/sdk.dart';
-
-final DartSdk sdk = new FolderBasedDartSdk(PhysicalResourceProvider.INSTANCE,
-    FolderBasedDartSdk.defaultSdkDirectory(PhysicalResourceProvider.INSTANCE))
-  ..useSummary = true;
-
-final DartSdk strongSdk = new FolderBasedDartSdk(PhysicalResourceProvider.INSTANCE,
-    FolderBasedDartSdk.defaultSdkDirectory(PhysicalResourceProvider.INSTANCE))
-  ..analysisOptions = (new AnalysisOptionsImpl()..strongMode = true)
-  ..useSummary = true;