Switch FE to use the libraries.json format.

pkg/front_end/lib/compiler_options.dart

Index: pkg/front_end/lib/compiler_options.dart
diff --git a/pkg/front_end/lib/compiler_options.dart b/pkg/front_end/lib/compiler_options.dart
index d20aa834fb8e1b727d9165ad2a3ba2dab69dadb1..c9b7f2da912ff8765c6301e838a069b70e447a2a 100644
--- a/pkg/front_end/lib/compiler_options.dart
+++ b/pkg/front_end/lib/compiler_options.dart
@@ -25,18 +25,17 @@ class CompilerOptions {
   /// [Platform.resolvedExecutable] as a starting point.
   Uri sdkRoot;
 
-  /// Map of `dart:*` libraries to URIs in the [fileSystem].
+  /// Uri to a platform libraries specification file.
   ///
-  /// Keys in the map are the name of the library with no `dart:` prefix, for
-  /// example:
-  ///
-  ///      {'core': 'file:///sdk/lib/core/core.dart'}
+  /// 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_spec.dart` for details on the
+  /// format.
   ///
-  /// 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;
+  /// 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 librariesSpecUri;
 
   /// Callback to which compilation errors should be delivered.
   ///