Add support for multi-roots

pkg/front_end/lib/src/multi_root_file_system.dart

Index: pkg/front_end/lib/src/multi_root_file_system.dart
diff --git a/pkg/front_end/lib/src/multi_root_file_system.dart b/pkg/front_end/lib/src/multi_root_file_system.dart
new file mode 100644
index 0000000000000000000000000000000000000000..12b084099af3f972c529116e065b9888c6f34ffc
--- /dev/null
+++ b/pkg/front_end/lib/src/multi_root_file_system.dart
@@ -0,0 +1,86 @@
+// 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.
+
+/// A file system that implements [CopilerOptions.multiRoots].
+library front_end.src.multi_roots_file_system;
+
+import 'dart:async';
+
+import 'package:front_end/file_system.dart';
+
+/// Wraps a file system to create an overlay of files from multiple roots.
+///
+/// Regular `file:` URIs are resolved directly in the underlying file system,
+/// but URIs that use a special [markerScheme] are resolved by searching
+/// under a set of given roots in order.
+///
+/// For example, consider the following inputs:
+///
+///   - markerScheme is `multi-root`
+///   - the set of roots are `file:///a/` and `file:///b/`
+///   - the underlying file system contains files:
+///         a/c/a1.dart
+///         a/c/a2.dart
+///         b/c/b1.dart
+///         b/c/a2.dart
+///         c/c1.dart
+///
+/// Then:
+///
+///   - file:///c/c1.dart is resolved as file:///c/c1.dart
+///   - multi-root:///c/a1.dart is resolved as file:///a/c/a1.dart
+///   - multi-root:///c/b1.dart is resolved as file:///b/c/b1.dart
+///   - multi-root:///c/a2.dart is resolved as file:///a/c/b2.dart
+class MultiRootFileSystem implements FileSystem {
+  final String markerScheme;
+  final List<Uri> roots;
+  final FileSystem original;
+
+  MultiRootFileSystem(this.markerScheme, List roots, this.original)
+      : roots = roots.map(_normalize).toList();
+
+  @override
+  FileSystemEntity entityForUri(Uri uri) =>
+      new MultiRootFileSystemEntity(this, uri);
+}
+
+/// Entity that searches the multiple roots and resolve a, possibly multi-root,
+/// entity to a plain entity under `multiRootFileSystem.original`.
+class MultiRootFileSystemEntity implements FileSystemEntity {
+  final MultiRootFileSystem multiRootFileSystem;
+  final Uri uri;
+  FileSystemEntity _delegate;
+  Future<FileSystemEntity> get delegate async =>
+      _delegate ??= await _resolveEntity();
+
+  Future<FileSystemEntity> _resolveEntity() async {
+    if (uri.scheme == multiRootFileSystem.markerScheme && uri.isAbsolute) {
+      var original = multiRootFileSystem.original;
+      assert(uri.path.startsWith('/'));
+      var path = uri.path.substring(1);
+      for (var root in multiRootFileSystem.roots) {
+        var candidate = original.entityForUri(root.resolve(path));
+        if (await candidate.exists()) return candidate;
+      }
+    }
+    return multiRootFileSystem.original.entityForUri(uri);
+  }
+
+  MultiRootFileSystemEntity(this.multiRootFileSystem, this.uri);
+
+  @override
+  Future<bool> exists() async => (await delegate).exists();
+
+  @override
+  Future<DateTime> lastModified() async => (await delegate).lastModified();
+
+  @override
+  Future<List<int>> readAsBytes() async => (await delegate).readAsBytes();
+
+  @override
+  Future<String> readAsString() async => (await delegate).readAsString();
+}
+
+_normalize(uri) =>
+    uri.path.endsWith('/') ? uri : uri.replace(path: '${uri.path}/');