Let printProgramText() to use filter to print only some libraries.

pkg/front_end/lib/src/fasta/kernel/utils.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 file.
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:front_end/src/scanner/token.dart' show Token;
 import 'package:kernel/ast.dart';
 import 'package:kernel/binary/ast_to_binary.dart';
 import 'package:kernel/text/ast_to_text.dart';
 
 /// A null-aware alternative to `token.offset`.  If [token] is `null`, returns
 /// `TreeNode.noOffset`.
 int offsetForToken(Token token) =>
     token == null ? TreeNode.noOffset : token.offset;
 
-/// Print the given [program].  Do nothing if it is `null`.
-void printProgramText(Program program) {
+/// Print the given [program].  Do nothing if it is `null`.  If the
+/// [libraryFilter] is provided, then only libraries that satisfy it are
+/// printed.
+void printProgramText(Program program, {bool libraryFilter(Library library)}) {
   if (program == null) return;
   StringBuffer sb = new StringBuffer();
   for (Library library in program.libraries) {
+    if (libraryFilter != null && !libraryFilter(library)) continue;
     Printer printer = new Printer(sb);
     printer.writeLibraryFile(library);
   }
   print(sb);
 }
 
 Future<Null> writeProgramToFile(Program program, Uri uri) async {
   File output = new File.fromUri(uri);
   IOSink sink = output.openWrite();
   try {
     new BinaryPrinter(sink).writeProgramFile(program);
     program.unbindCanonicalNames();
   } finally {
     await sink.close();
   }
 }