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

pkg/front_end/lib/compiler_options.dart

 // 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.
 
 library front_end.compiler_options;
 
 import 'package:front_end/src/base/performace_logger.dart';
 import 'package:front_end/src/incremental/byte_store.dart';
 import 'package:kernel/target/targets.dart' show Target;
 
 import 'compilation_message.dart';
 import 'file_system.dart';
 import 'physical_file_system.dart';
 
 /// Callback used to report errors encountered during compilation.
 typedef void ErrorHandler(CompilationMessage error);
 
 /// Front-end options relevant to compiler back ends.
 ///
 /// Not intended to be implemented or extended by clients.
 class CompilerOptions {
   /// The URI of the root of the Dart SDK (typically a "file:" URI).
   ///
   /// If `null`, the SDK will be searched for using
   /// [Platform.resolvedExecutable] as a starting point.
   Uri sdkRoot;
 
-  /// Uri to a platform libraries specification file.
+  /// Map of `dart:*` libraries to URIs in the [fileSystem].
   ///
-  /// A libraries specification file is a JSON file that describes how to map
-  /// `dart:*` libraries to URIs in the underlying [fileSystem].  See
-  /// `package:front_end/src/base/libraries_specification.dart` for details on
-  /// the format.
+  /// Keys in the map are the name of the library with no `dart:` prefix, for
+  /// example:
   ///
-  /// If a value is not specified and `compileSdk = true`, the compiler will
-  /// infer at a default location under [sdkRoot], typically under
-  /// `lib/libraries.json`.
-  Uri librariesSpecificationUri;
+  ///      {'core': 'file:///sdk/lib/core/core.dart'}
+  ///
+  /// If `null`, the default set of libraries will be loaded from
+  /// `sdkRoot/lib/libraries.json`.
+  // TODO(sigmund): also provide an option to specify the .json file, then
+  // consider dropping this option.
+  Map<String, Uri> dartLibraries;
 
   /// Callback to which compilation errors should be delivered.
   ///
   /// By default, when no callback is provided, the compiler will report
   /// messages on the console and will throw when fatal errors are discovered.
   ErrorHandler onError;
 
   /// Whether messages should be reported using the compiler's internal
   /// reporting mechanism.
   ///
@@ skipping 176 matching lines @@
   ///
   /// Typically used by developers to debug internals of the compiler.
   bool throwOnWarnings = false;
 
   /// Whether the compiler should throw as soon as it encounters a
   /// compilation nit.
   ///
   /// Typically used by developers to debug internals of the compiler.
   bool throwOnNits = false;
 }