Revert "Switch FE to use the libraries.json format."

pkg/front_end/test/src/incremental/mock_sdk.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 'package:front_end/memory_file_system.dart';
-import 'package:front_end/src/base/libraries_specification.dart';
 
 /// Create SDK libraries which are used by Fasta to perform kernel generation.
-/// The root of the SDK is `file:///sdk`, it will contain a libraries
-/// specification file at `lib/libraries.json`.
-///
-/// Returns the [TargetLibrariesSpecification] whose contents are in
-/// libraries.json.
-TargetLibrariesSpecification createSdkFiles(MemoryFileSystem fileSystem) {
-  Map<String, LibraryInfo> dartLibraries = {};
+/// Return the mapping from the simple names of these library to the URIs
+/// in the given [fileSystem].  The root of the SDK is `file:///sdk`.
+Map<String, Uri> createSdkFiles(MemoryFileSystem fileSystem) {
+  Map<String, Uri> dartLibraries = {};
+
   void addSdkLibrary(String name, String contents) {
     String path = '$name/$name.dart';
     Uri uri = Uri.parse('file:///sdk/lib/$path');
     fileSystem.entityForUri(uri).writeAsStringSync(contents);
-    dartLibraries[name] = new LibraryInfo(name, uri, const []);
+    dartLibraries[name] = uri;
   }
 
   addSdkLibrary('core', r'''
 library dart.core;
 import 'dart:_internal';
 import 'dart:async';
 
 class Object {
   const Object();
   bool operator ==(other) => identical(this, other);
@@ skipping 230 matching lines @@
   addSdkLibrary('_internal', '''
 library dart._internal;
 class Symbol {}
 class ExternalName {
   final String name;
   const ExternalName(this.name);
 }
 ''');
   addSdkLibrary('_vmservice', 'library dart._vmservice;');
 
-  var targetSpec = new TargetLibrariesSpecification('vm', dartLibraries);
-  var spec = new LibrariesSpecification({'vm': targetSpec});
-
-  Uri uri = Uri.parse('file:///sdk/lib/libraries.json');
-  fileSystem.entityForUri(uri).writeAsStringSync(spec.toJsonString(uri));
-  return targetSpec;
+  return dartLibraries;
 }