Use messages for (some) public API errors

pkg/front_end/test/subpackage_relationships_test.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/compiler_options.dart';
 import 'package:front_end/dependency_grapher.dart';
 import 'package:path/path.dart' as pathos;
@@ skipping 23 matching lines @@
 ///
 /// Each listed directory is considered a subpackage.  Each package contains all
 /// of its descendant files that are not in a more deeply nested subpackage.
 ///
 /// TODO(paulberry): stuff in lib/src shouldn't depend on lib; lib should just
 /// re-export stuff in lib/src.
 final subpackageRules = {
   'lib': new SubpackageRules(allowedDependencies: [
     'lib/src',
     'lib/src/base',
+    'lib/src/fasta',
     'lib/src/incremental',
   ]),
   'lib/src': new SubpackageRules(allowedDependencies: [
     'lib',
     'lib/src/base',
     'lib/src/fasta',
     "lib/src/fasta/dill",
     "lib/src/fasta/kernel",
     'lib/src/fasta/source',
     'lib/src/incremental',
@@ skipping 260 matching lines @@
       if (subpackageRules[subpackage].allowSubdirs) {
         subpackageRules[subpackage].actuallyHasSubdirs = true;
       } else {
         problem('Uri $src is in a subfolder of $subpackage, but that '
             'subpackage does not allow dart files in subdirectories.');
       }
     }
     return subpackage;
   }
 }