Extend front_end API to accept patch files

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 'compilation_error.dart';
 import 'file_system.dart';
 import 'physical_file_system.dart';
 
@@ skipping 67 matching lines @@
   /// Any Uri that resolves to "multi-root:///$rest" will be searched for
   /// at "$root/$rest", where "$root" is drawn from this list.
   ///
   /// Intended use: if the user has a Bazel workspace located at path
   /// "$workspace", this could be set to the file URIs corresponding to the
   /// paths for "$workspace", "$workspace/bazel-bin",
   /// and "$workspace/bazel-genfiles", effectively overlaying source and
   /// generated files.
   List<Uri> multiRoots = [];
 
-  /// Sets the platform bit, which determines which patch files should be
-  /// applied to the SDK.
-  ///
-  /// The value should be a power of two, and should match the `PLATFORM` bit
-  /// flags in sdk/lib/_internal/sdk_library_metadata/lib/libraries.dart.  If
-  /// zero, no patch files will be applied.
-  int platformBit;
-
   /// The declared variables for use by configurable imports and constant
   /// evaluation.
   Map<String, String> declaredVariables;
 
   /// The [FileSystem] which should be used by the front end to access files.
   ///
   /// All file system access performed by the front end goes through this
   /// mechanism, with one exception: if no value is specified for
   /// [packagesFileUri], the packages file is located using the actual physical
   /// file system.  TODO(paulberry): fix this.
   FileSystem fileSystem = PhysicalFileSystem.instance;
 
   /// Whether to generate code for the SDK when compiling a whole-program.
   bool compileSdk = false;
 
   /// Whether a modular build compiles only the files listed explicitly or if it
   /// compiles dependencies as well.
   ///
   /// This option is intended only for modular APIs like `kernelForBuildUnit`.
   /// These APIs by default ensure that builds are hermetic, where all files
   /// that will be compiled are listed explicitly and all other dependencies
   /// are covered by summary files.
   ///
   /// When this option is true, these APIs will treat any dependency that is
   /// not described in a summary as if it was explictly listed as an input.
   bool chaseDependencies = false;
 
   /// Whether to intepret Dart sources in strong-mode.
   bool strongMode = true;
+
+  // All options below are target-specific options.
+  //
+  // TODO(sigmund): revisit the right layout for these options. We might want to
+  // split them out into a separate bag of options or provide factories for
+  // common combinations of these options.
+
+  /// Patch files to apply on the core libraries for a specific target platform.
+  ///
+  /// Keys on this map are expected to be `dart:*` URIs. The values can be
+  /// either absolute or relative URIs. Absolute URIs are read directly, while
+  /// relative URIs are resolved from the [sdkRoot].
+  Map<Uri, List<Uri>> targetPatches = {};
 }