Rework DillLoader to allow adding multiple dills.

pkg/front_end/test/fasta/testing/suite.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.md file.
 
 library fasta.testing.suite;
 
 import 'dart:async' show Future;
 
 import 'dart:io' show File;
 
@@ skipping 175 matching lines @@
     return fullCompile ? "${astKind} compile" : "outline";
   }
 
   bool get isCompiler => fullCompile;
 
   Future<Result<Program>> run(
       TestDescription description, FastaContext 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.setProgram(platform);
     KernelTarget sourceTarget = astKind == AstKind.Analyzer
         ? new AnalyzerTarget(dillTarget, context.uriTranslator, strongMode)
         : new KernelTarget(PhysicalFileSystem.instance, dillTarget,
             context.uriTranslator, strongMode);
 
     Program p;
     try {
       sourceTarget.read(description.uri);
       await dillTarget.writeOutline(null);
       ValidatingInstrumentation instrumentation;
@@ skipping 13 matching lines @@
             return fail(null, instrumentation.problemsAsString);
           }
         }
       }
     } on InputError catch (e, s) {
       return fail(null, e.error, s);
     }
     return pass(p);
   }
 }