Rework DillLoader to allow adding multiple dills.

pkg/kernel/test/closures/suite.dart

 // Copyright (c) 2017, 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.
 
 library test.kernel.closures.suite;
 
 import 'dart:async' show Future;
 
 import 'package:front_end/physical_file_system.dart';
 import 'package:testing/testing.dart'
@@ skipping 75 matching lines @@
     extends Step<TestDescription, Program, ClosureConversionContext> {
   const FastaCompile();
 
   String get name => "fasta compilation";
 
   Future<Result<Program>> run(
       TestDescription description, ClosureConversionContext context) async {
     Program platform = await context.loadPlatform();
     Ticker ticker = new Ticker();
     DillTarget dillTarget = new DillTarget(ticker, context.uriTranslator);
-    dillTarget.loader
-      ..input = Uri.parse("org.dartlang:platform") // Make up a name.
-      ..setProgram(platform);
+    dillTarget.loader.appendLibraries(platform);
     KernelTarget sourceTarget = new KernelTarget(PhysicalFileSystem.instance,
         dillTarget, context.uriTranslator, context.strongMode);
 
     Program p;
     try {
       sourceTarget.read(description.uri);
       await dillTarget.writeOutline(null);
       await sourceTarget.writeOutline(null);
       p = await sourceTarget.writeProgram(null);
     } on InputError catch (e, s) {
@@ skipping 14 matching lines @@
     try {
       program = closure_conversion.transformProgram(program);
       return pass(program);
     } catch (e, s) {
       return crash(e, s);
     }
   }
 }
 
 main(List<String> arguments) => runMe(arguments, createContext, "testing.json");