Rework DillLoader to allow adding multiple dills.

pkg/front_end/lib/src/fasta/dill/dill_target.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 file.
 
 library fasta.dill_target;
 
 import 'dart:async' show Future;
 
 import 'package:kernel/ast.dart' show Class;
 
@@ skipping 19 matching lines @@
       : super(ticker, uriTranslator) {
     loader = new DillLoader(this);
   }
 
   void addSourceInformation(
       Uri uri, List<int> lineStarts, List<int> sourceCode) {
     internalError("Unsupported operation.");
   }
 
   void read(Uri uri) {
-    if (loader.input == null) {
-      loader.input = uri;
-    } else {
-      inputError(uri, -1, "Can only read one dill file.");
-    }
+    internalError("not implemented");

[ahe, 2017/05/11 10:09:49]

"Internal error: Unsupported operation."

   }
 
   Future<Null> writeProgram(Uri uri) {
     return internalError("not implemented.");
   }
 
   Future<Null> writeOutline(Uri uri) async {
-    if (loader.input == null) return null;
+    if (loader.program == null) return null;
     await loader.buildOutlines();
     isLoaded = true;
     return null;
   }
 
   DillLibraryBuilder createLibraryBuilder(Uri uri, Uri fileUri) {
     return new DillLibraryBuilder(uri, loader);
   }
 
   void addDirectSupertype(ClassBuilder cls, Set<ClassBuilder> set) {}
 
   List<ClassBuilder> collectAllClasses() {
     return null;
   }
 
   void breakCycle(ClassBuilder cls) {}
 
   Class get objectClass => loader.coreLibrary["Object"].target;
 }