Read file names from libraries.json, not URIs.

pkg/front_end/lib/src/fasta/translate_uri.dart

Index: pkg/front_end/lib/src/fasta/translate_uri.dart
diff --git a/pkg/front_end/lib/src/fasta/translate_uri.dart b/pkg/front_end/lib/src/fasta/translate_uri.dart
index 6c249a77239f69aa4782097213ae1e4208243038..095dd2a10c4b3dc7c312a7867f29e385a9692796 100644
--- a/pkg/front_end/lib/src/fasta/translate_uri.dart
+++ b/pkg/front_end/lib/src/fasta/translate_uri.dart
@@ -98,7 +98,7 @@ Future<Map<String, Uri>> computeLibraries(
       .decode(await fileSystem.entityForUri(uri).readAsString())["libraries"];
   Map<String, Uri> result = <String, Uri>{};
   libraries.forEach((String name, String path) {
-    result[name] = uri.resolve(path);
+    result[name] = uri.resolveUri(new Uri.file(path));
   });
   return result;
 }