Don't crash on missing dart: URI.

pkg/fasta/lib/src/translate_uri.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.translate_uri;
 
 import 'dart:async' show
     Future;
 
 import 'dart:io' show
     File;
 
 import 'package:package_config/packages_file.dart' as packages_file show
     parse;
 
-import 'errors.dart' show
-    internalError;
-
 class TranslateUri {
   final Map<String, Uri> packages;
 
   TranslateUri(this.packages);
 
   Uri translate(Uri uri) {
     if (uri.scheme == "dart") return translateDartUri(uri);
     if (uri.scheme == "package") return translatePackageUri(uri);
     return null;
   }
 
-  Uri translateDartUri(Uri uri) {
-    throw internalError("dart: URIs not implemented yet.");
-  }
+  // TODO(ahe): Implement loading platform file on demand. This method must
+  // become async for that to happen.
+  Uri translateDartUri(Uri uri) => null;
 
   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);
   }
 
   static Future<TranslateUri> parse([Uri uri]) async {
     uri ??= Uri.base.resolve(".packages");
     File file = new File.fromUri(uri);
     List<int> bytes = await file.readAsBytes();
     Map<String, Uri> packages = packages_file.parse(bytes, uri);
     return new TranslateUri(packages);
   }
 }