Don't attempt to recover from missing strings.

pkg/fasta/testing.json

 {
 "":"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.md file.",
   "packages": "../../.packages",
   "suites": [
     {
       "name": "scanner",
       "kind": "Chain",
       "source": "../dart_scanner/test/scanner_suite.dart",
@@ skipping 94 matching lines @@
     "uris": [
       "bin",
       "lib",
       "lib/src",
       "lib/src/analyzer",
       "lib/src/builder",
       "lib/src/dill",
       "lib/src/kernel",
       "lib/src/source",
       "test/outline_test.dart",
+      "test/dart2js_test.dart",
       "../dart_scanner/bin",
       "../dart_scanner/lib",
       "../dart_scanner/lib/src",
       "../dart_scanner/lib/testing",
       "../dart_scanner/test",
       "../dart_parser/bin",
       "../dart_parser/lib",
       "../dart_parser/lib/src",
       "../dart_parser/test",
       "../compiler/lib/src/dart2js.dart"
     ],
     "exclude": [
       "/third_party/pkg/"
     ]
   }
 }