Reapply "Add "load from .dill" file capability and run a white-box test.""

tests/compiler/dart2js/analyze_unused_dart2js_test.dart

Index: tests/compiler/dart2js/analyze_unused_dart2js_test.dart
diff --git a/tests/compiler/dart2js/analyze_unused_dart2js_test.dart b/tests/compiler/dart2js/analyze_unused_dart2js_test.dart
index c73aee5dbe7e833f345179491ce97502efb2604d..82f934e283bfa08c8c1cabac0784de5ce682f26d 100644
--- a/tests/compiler/dart2js/analyze_unused_dart2js_test.dart
+++ b/tests/compiler/dart2js/analyze_unused_dart2js_test.dart
@@ -8,6 +8,7 @@ import 'package:async_helper/async_helper.dart';
 
 import 'package:compiler/src/compiler.dart';
 import 'package:compiler/src/diagnostics/messages.dart';
+import 'package:compiler/src/elements/elements.dart' show LibraryElement;
 import 'package:compiler/src/filenames.dart';
 
 import 'analyze_helper.dart';
@@ -80,6 +81,7 @@ bool checkResults(Compiler compiler, CollectingDiagnosticHandler handler) {
     }
   }
 
-  compiler.libraryLoader.lookupLibrary(helperUri).forEachLocalMember(checkLive);
+  (compiler.libraryLoader.lookupLibrary(helperUri) as LibraryElement)
+      .forEachLocalMember(checkLive);
   return handler.checkResults();
 }