Set fileUri when creating library builders.

pkg/front_end/lib/src/fasta/loader.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.loader;
 
 import 'dart:async' show
     Future;
 
 import 'dart:collection' show
@@ skipping 29 matching lines @@
   LibraryBuilder first;
 
   int byteCount = 0;
 
   Uri currentUriForCrashReporting;
 
   Loader(this.target);
 
   Ticker get ticker => target.ticker;
 
-  LibraryBuilder read(Uri uri) {
+  /// Look up a library builder by the the name [uri], or if such doesn't
+  /// exist, create one. The canonical URI of the library is [uri], and its
+  /// actual location is [fileUri].
+  ///
+  /// Canonical URIs have schemes like "dart", or "package", and the actual
+  /// location is often a file URI.
+  LibraryBuilder read(Uri uri, [Uri fileUri]) {
     firstSourceUri ??= uri;
     LibraryBuilder builder = builders.putIfAbsent(uri, () {
-      LibraryBuilder library = target.createLibraryBuilder(uri);
+      if (fileUri == null) {
+        switch (uri.scheme) {
+          case "package":
+          case "dart":
+            fileUri = target.translateUri(uri);
+            break;
+
+          default:
+            fileUri = uri;
+            break;
+        }
+      }
+      LibraryBuilder library = target.createLibraryBuilder(uri, fileUri);
       if (uri.scheme == "dart" && uri.path == "core") {
         coreLibrary = library;
       }
       first ??= library;
       if (library.loader == this) {
         unparsedLibraries.addLast(library);
       }
       return library;
     });
     return builder;
@@ skipping 59 matching lines @@
         errors.addAll(library.compileTimeErrors);
       }
     }
     return errors;
   }
 }
 
 String format(double d, int fractionDigits, int width) {
   return d.toStringAsFixed(fractionDigits).padLeft(width);
 }