Rework DillLoader to allow adding multiple dills.

pkg/front_end/lib/src/fasta/dill/dill_loader.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_loader;
 
 import 'dart:async' show Future;
 
-import 'dart:io' show File;
-
-import 'package:kernel/kernel.dart' show loadProgramFromBinary;
-
 import 'package:kernel/ast.dart' show Library, Program;
 
 import '../loader.dart' show Loader;
-
 import '../target_implementation.dart' show TargetImplementation;
-
 import 'dill_library_builder.dart' show DillLibraryBuilder;
 
 class DillLoader extends Loader<Library> {
-  Uri input;
-
   Program program;
 
   DillLoader(TargetImplementation target) : super(target);
 
-  DillLibraryBuilder read(Uri uri, [Uri fileUri]) => super.read(uri, fileUri);
-
-  Future<Null> buildOutline(DillLibraryBuilder builder) async {
-    if (program == null) {
-      byteCount = await new File.fromUri(input).length();
-      setProgram(await loadProgramFromBinary(input.toFilePath()));
+  /// Append compiled libraries from the given [program]. If the [filter] is
+  /// provided, append only libraries whose [Uri] is accepted by the [filter].
+  void appendLibraries(Program program, [bool filter(Uri uri)]) {
+    this.program ??= new Program();
+    program.unbindCanonicalNames();
+    for (Library library in program.libraries) {
+      if (filter == null || filter(library.importUri)) {
+        this.program.libraries.add(library);
+        read(library.importUri).library = library;
+      }
     }
-    builder.library.classes.forEach(builder.addClass);
-    builder.library.procedures.forEach(builder.addMember);
-    builder.library.fields.forEach(builder.addMember);
   }
 
   Future<Null> buildBody(DillLibraryBuilder builder) {
     return buildOutline(builder);
   }
 
-  void setProgram(Program program) {
-    assert(input != null);
-    this.program = program;
-    program.unbindCanonicalNames();
-    for (Library library in program.libraries) {
-      read(library.importUri).library = library;
-    }
+  Future<Null> buildOutline(DillLibraryBuilder builder) async {
+    builder.library.classes.forEach(builder.addClass);
+    builder.library.procedures.forEach(builder.addMember);
+    builder.library.fields.forEach(builder.addMember);
   }
+
+  DillLibraryBuilder read(Uri uri, [Uri fileUri]) => super.read(uri, fileUri);
 }