Rework DillLoader to allow adding multiple dills.

pkg/kernel/lib/kernel.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.
 
 /// Conventions for paths:
 ///
 /// - Use the [Uri] class for paths that may have the `file`, `dart` or
 ///   `package` scheme.  Never use [Uri] for relative paths.
 /// - Use [String]s for all filenames and paths that have no scheme prefix.
 /// - Never translate a `dart:` or `package:` URI into a `file:` URI, instead
 ///   translate it to a [String] if the file system path is needed.
 /// - Only use [File] from dart:io at the last moment when it is needed.
 ///
 library kernel;
 
 import 'ast.dart';
 import 'binary/ast_to_binary.dart';
 import 'binary/ast_from_binary.dart';
 import 'dart:async';
 import 'dart:io';
 import 'text/ast_to_text.dart';
 
 export 'ast.dart';
 
 Program loadProgramFromBinary(String path, [Program program]) {
+  List<int> bytes = new File(path).readAsBytesSync();
+  return loadProgramFromBytes(bytes, program);
+}
+
+Program loadProgramFromBytes(List<int> bytes, [Program program]) {
   program ??= new Program();
-  new BinaryBuilder(new File(path).readAsBytesSync()).readProgram(program);
+  new BinaryBuilder(bytes).readProgram(program);
   return program;
 }
 
 Future writeProgramToBinary(Program program, String path) {
   var sink;
   if (path == 'null' || path == 'stdout') {
     sink = stdout.nonBlocking;
   } else {
     sink = new File(path).openWrite();
   }
@@ skipping 26 matching lines @@
     {String path, bool showExternal: false, bool showOffsets: false}) {
   StringBuffer buffer = new StringBuffer();
   new Printer(buffer, showExternal: showExternal, showOffsets: showOffsets)
       .writeProgramFile(program);
   if (path == null) {
     print(buffer);
   } else {
     new File(path).writeAsStringSync('$buffer');
   }
 }