Rename TranslateUri to UriTranslator, rename files, extract Impl.

pkg/front_end/lib/src/fasta/uri_translator.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 fasta.uri_translator;
+
+import 'package:front_end/file_system.dart' show FileSystem;
+
+/// Instances of [UriTranslator] translate absolute URIs into corresponding
+/// file URIs in a [FileSystem]. Translated URIs are typically `file:` URIs,
+/// but may use a different scheme depending on the used custom file system.
+abstract class UriTranslator {
+  /// Return the URIs of patches that should be applied to the platform library
+  /// with the given [libraryName], or `null` if there are no patches to apply.

[Siggi Cherem (dart-lang), 2017/07/12 17:55:23]

alternatively, we could drop the "or `null`..." and return a const<Uri>[]
instead.

[scheglov, 2017/07/12 18:53:14]

Yeah, I thought about this.
But then there is TODO in impl class, and if we implement it, we would still
have to return null.

  // TODO(ahe): We probably want this to be `Map<String, Uri>`, that is, just
  // one patch library (with parts).

+  List<Uri> getDartPatches(String libraryName);
+
+  /// Returns `true` if [uri] is private to the platform libraries (and thus
+  /// not accessible from user code).
+  bool isPlatformImplementation(Uri uri);
+
+  /// Return the corresponding file URI for the given absolute [uri], or `null`
+  /// if there is no corresponding file URI, or the given [uri] is already a
+  /// file URI.
+  ///
+  /// This is the URIs only transformation, there is no guarantee that the

[Siggi Cherem (dart-lang), 2017/07/12 17:55:24]

nit: just to make it easier to read:

"This is the URIs only transformation, ..."
 => 
"Note: this only translates the URI, ...

[scheglov, 2017/07/12 18:53:14]

Done.

+  /// corresponding file exists in the file system.
+  Uri translate(Uri uri);
+}