Add support for multi-roots

pkg/front_end/lib/src/base/processed_options.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 'dart:async';
 
 import 'package:kernel/target/targets.dart';
 import 'package:kernel/target/vm_fasta.dart';
 import 'package:front_end/compiler_options.dart';
 import 'package:front_end/file_system.dart';
+import 'package:front_end/src/base/performace_logger.dart';
+import 'package:front_end/src/fasta/errors.dart';
+import 'package:front_end/src/fasta/ticker.dart';
 import 'package:front_end/src/fasta/translate_uri.dart';
-import 'package:front_end/src/fasta/ticker.dart';
-import 'package:front_end/src/fasta/errors.dart';
-import 'package:front_end/src/base/performace_logger.dart';
 import 'package:front_end/src/incremental/byte_store.dart';
+import 'package:front_end/src/multi_root_file_system.dart';
 import 'package:front_end/src/simple_error.dart';
 import 'package:package_config/packages_file.dart' as package_config;
 import 'package:kernel/kernel.dart'
     show Program, loadProgramFromBytes, CanonicalName;
 
 /// All options needed for the front end implementation.
 ///
 /// This includes: all of [CompilerOptions] in a form useful to the
 /// implementation, default values for options that were not provided,
 /// and information derived from how the compiler was invoked (like the
@@ skipping 245 matching lines @@
 
     // Infer based on the sdkRoot, but only when `compileSdk` is false,
     // otherwise the default intent was to compile the sdk from sources and not
     // to load an sdk summary file.
     if (_raw.compileSdk) return null;
     return sdkRoot.resolve('outline.dill');
   }
 
   /// Create a [FileSystem] specific to the current options.
   ///
-  /// If [chaseDependencies] is false, the resulting file system will be
-  /// hermetic.
+  /// If `_raw.multiRoots` is not empty, the file-system will implement the
+  /// semantics of multiple roots. If [chaseDependencies] is false, the
+  /// resulting file system will be hermetic.
   FileSystem _createFileSystem() {
     var result = _raw.fileSystem;
+    // Note: hermetic checks are done before translating multi-root URIs, so
+    // the order in which we create the file systems below is relevant.
+    if (!_raw.multiRoots.isEmpty) {
+      result = new MultiRootFileSystem('multi-root', _raw.multiRoots, result);
+    }
     if (!chaseDependencies) {
       var allInputs = inputs.toSet();
       allInputs.addAll(_raw.inputSummaries);
       allInputs.addAll(_raw.linkedDependencies);
 
       if (sdkSummary != null) allInputs.add(sdkSummary);
 
       if (_raw.sdkRoot != null) {
         // TODO(sigmund): refine this, we should be more explicit about when
         // sdkRoot and libraries.json are allowed to be used.
         allInputs.add(sdkRoot);
         allInputs.add(sdkRoot.resolve("lib/libraries.json"));
       }
 
       /// Note: Searching the file-system for the package-config is not
       /// supported in hermetic builds.
       if (_raw.packagesFileUri != null) allInputs.add(_raw.packagesFileUri);
       result = new HermeticFileSystem(allInputs, result);
     }
-    // TODO(paulberry): support multiRoots.
-    assert(_raw.multiRoots.isEmpty);
     return result;
   }
 }
 
 /// A [FileSystem] that only allows access to files that have been explicitly
 /// whitelisted.
 class HermeticFileSystem implements FileSystem {
   final Set<Uri> includedFiles;
   final FileSystem _realFileSystem;
 
   HermeticFileSystem(this.includedFiles, this._realFileSystem);
 
   FileSystemEntity entityForUri(Uri uri) {
     if (includedFiles.contains(uri)) return _realFileSystem.entityForUri(uri);
     return inputError(
         null,
         -1,
         'Invalid access to $uri: '
         'the file is accessed in a modular hermetic build '
         '(where chaseDependencies is false), but it was not '
         'explicitly listed as an input.');
   }
 }