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

pkg/front_end/lib/src/fasta/kernel/kernel_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.kernel_target;
 
 import 'dart:async' show Future;
 
 import 'dart:io' show File;
 
@@ skipping 677 matching lines @@
     var toShake =
         dillTarget.loader.libraries.map((lib) => lib.importUri).toSet();
     var isIncluded = (Uri uri) => !toShake.contains(uri);
     var data = new RetainedDataBuilder();
     // TODO(sigmund): replace this step with data that is directly computed from
     // the builders: we should know the tree-shaking roots without having to do
     // a second visit over the tree.
     new RootsMarker(data).run(_program, isIncluded);
     trimProgram(_program, data, isIncluded);
   }
+
+  /// Return `true` if the given [library] was built by this [KernelTarget]
+  /// from sources, and not loaded from a [DillTarget].
+  bool isSourceLibrary(Library library) {
+    return loader.libraries.contains(library);
+  }
 }
 
 /// Looks for a constructor call that matches `super()` from a constructor in
 /// [cls]. Such a constructor may have optional arguments, but no required
 /// arguments.
 Constructor defaultSuperConstructor(Class cls) {
   Class superclass = cls.superclass;
   while (superclass != null && superclass.isMixinApplication) {
     superclass = superclass.superclass;
   }
   for (Constructor constructor in superclass.constructors) {
     if (constructor.name.name.isEmpty) {
       return constructor.function.requiredParameterCount == 0
           ? constructor
           : null;
     }
   }
   return null;
 }