support resolving .packages in FE

pkg/front_end/lib/src/fasta/uri_translator_impl.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_impl;
 
 import 'dart:async' show Future;
 import 'dart:convert' show JSON;
 
 import 'package:front_end/file_system.dart'
     show FileSystem, FileSystemException;
 import 'package:front_end/src/fasta/uri_translator.dart';
 import 'package:package_config/packages_file.dart' as packages_file show parse;
+import 'package:package_config/packages.dart' show Packages;
+import 'package:package_config/src/packages_impl.dart' show MapPackages;
 
 import 'deprecated_problems.dart' show deprecated_inputError;
 
 /// Read the JSON file with defined SDK libraries from the given [uri] in the
 /// [fileSystem] and return the mapping from parsed Dart library names (e.g.
 /// `math`) to file URIs.
 Future<Map<String, Uri>> computeDartLibraries(
     FileSystem fileSystem, Uri uri) async {
   if (uri == null) return const <String, Uri>{};
   Map<String, String> libraries = JSON
@@ skipping 15 matching lines @@
 class UriTranslatorImpl implements UriTranslator {
   /// Mapping from Dart library names (e.g. `math`) to file URIs.
   final Map<String, Uri> dartLibraries;
 
   // TODO(ahe): We probably want this to be `Map<String, Uri>`, that is, just
   // one patch library (with parts).
   /// Mapping from Dart library names to the file URIs of patches to apply.
   final Map<String, List<Uri>> dartPatches;
 
   /// Mapping from package names (e.g. `angular`) to the file URIs.
-  final Map<String, Uri> packages;
+  final Packages packages;
 
   UriTranslatorImpl(this.dartLibraries, this.dartPatches, this.packages);
 
   @override
   List<Uri> getDartPatches(String libraryName) => dartPatches[libraryName];
 
   @override
   bool isPlatformImplementation(Uri uri) {
     if (uri.scheme != "dart") return false;
     String path = uri.path;
@@ skipping 19 matching lines @@
     String libraryName = path.substring(0, index);
     String relativePath = path.substring(index + 1);
     Uri libraryFileUri = dartLibraries[libraryName];
     return libraryFileUri?.resolve(relativePath);
   }
 
   /// Return the file URI that corresponds to the given `package` URI, or
   /// `null` if the `package` [uri] format is invalid, or there is no
   /// corresponding package registered.
   Uri _translatePackageUri(Uri uri) {
-    int index = uri.path.indexOf("/");
-    if (index == -1) return null;
-    String name = uri.path.substring(0, index);
-    String path = uri.path.substring(index + 1);
-    Uri root = packages[name];
-    if (root == null) return null;
-    return root.resolve(path);
+    try {
+      // TODO(sigmund): once we remove the `parse` API, we can ensure that
+      // packages will never be null and get rid of `?` below.
+      return packages?.resolve(uri, notFound: (_) => null);

[ahe, 2017/07/14 13:44:57]

Handle not found by reporting an error?

[Siggi Cherem (dart-lang), 2017/07/14 19:54:06]

Done.

Note I was preserving the old semantics here: if a package was not declared in
.packages, we were returning null here and reporting an error later when loading
the file.

I added the error here, but it is possible that we have some double-reporting
going on... we need to follow up on that. Added a TODO

+    } on ArgumentError catch (_) {

[ahe, 2017/07/14 13:44:58]

Argument error can occur for many reasons. Why is it safe to catch it here?

[Siggi Cherem (dart-lang), 2017/07/14 19:54:06]

I hear you. As discussed offline, I filed a bug on the package_config package to
use a more specific error object:
https://github.com/dart-lang/package_config/issues/40

+      // TODO(sigmund): report an error.

[Siggi Cherem (dart-lang), 2017/07/14 04:58:52]

(I plan to do so once Peter lands his CL adding context.report)

+      return null;
+    }
   }
 
   static Future<UriTranslator> parse(FileSystem fileSystem, Uri sdk,
       {Uri packages}) async {
     Uri librariesJson = sdk?.resolve("lib/libraries.json");
 
     // TODO(ahe): Provide a value for this file.
     Uri patches = null;
 
     packages ??= Uri.base.resolve(".packages");
 
     List<int> bytes;
     try {
       bytes = await fileSystem.entityForUri(packages).readAsBytes();
     } on FileSystemException catch (e) {
       deprecated_inputError(packages, -1, e.message);
     }
 
-    Map<String, Uri> parsedPackages;
+    Packages parsedPackages;
     try {
-      parsedPackages = packages_file.parse(bytes, packages);
+      parsedPackages = new MapPackages(packages_file.parse(bytes, packages));
     } on FormatException catch (e) {
       return deprecated_inputError(packages, e.offset, e.message);
     }
 
     var dartLibraries = await computeDartLibraries(fileSystem, librariesJson);
     return new UriTranslatorImpl(dartLibraries,
         await computeDartPatches(fileSystem, patches), parsedPackages);
   }
 }