Make pub strong-mode clean.

lib/src/pubspec.dart

 // Copyright (c) 2012, 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 'package:path/path.dart' as path;
 import 'package:pub_semver/pub_semver.dart';
 import 'package:source_span/source_span.dart';
 import 'package:yaml/yaml.dart';
 
 import 'barback/transformer_config.dart';
@@ skipping 133 matching lines @@
     if (transformers == null) {
       _transformers = [];
       return _transformers;
     }
 
     if (transformers is! List) {
       _error('"transformers" field must be a list.',
           fields.nodes['transformers'].span);
     }
 
-    _transformers = transformers.nodes.map((phase) {
+    _transformers = (transformers as YamlList).nodes.map((phase) {
       var phaseNodes = phase is YamlList ? phase.nodes : [phase];
       return phaseNodes.map((transformerNode) {
         var transformer = transformerNode.value;
         if (transformer is! String && transformer is! Map) {
           _error('A transformer must be a string or map.',
                  transformerNode.span);
         }
 
         var libraryNode;
         var configurationNode;
@@ skipping 256 matching lines @@
         '"$expectedName".', this.fields.nodes["name"].span);
   }
 
   /// Parses the pubspec stored at [filePath] whose text is [contents].
   ///
   /// If the pubspec doesn't define a version for itself, it defaults to
   /// [Version.none].
   factory Pubspec.parse(String contents, SourceRegistry sources,
       {String expectedName, Uri location}) {
     var pubspecNode = loadYamlNode(contents, sourceUrl: location);
+    Map pubspecMap;
     if (pubspecNode is YamlScalar && pubspecNode.value == null) {
-      pubspecNode = new YamlMap(sourceUrl: location);
-    } else if (pubspecNode is! YamlMap) {
+      pubspecMap = new YamlMap(sourceUrl: location);
+    } else if (pubspecNode is YamlMap) {
+      pubspecMap = pubspecNode;
+    } else {
       throw new PubspecException(
           'The pubspec must be a YAML mapping.', pubspecNode.span);
     }
 
-    return new Pubspec.fromMap(pubspecNode, sources,
+    return new Pubspec.fromMap(pubspecMap, sources,
         expectedName: expectedName, location: location);
   }
 
   /// Returns a list of most errors in this pubspec.
   ///
   /// This will return at most one error for each field.
   List<PubspecException> get allErrors {
     var errors = <PubspecException>[];
     _getError(fn()) {
       try {
@@ skipping 166 matching lines @@
     implements ApplicationException {
   PubspecException(String message, SourceSpan span)
       : super(message, span);
 }
 
 /// Returns whether [uri] is a file URI.
 ///
 /// This is slightly more complicated than just checking if the scheme is
 /// 'file', since relative URIs also refer to the filesystem on the VM.
 bool _isFileUri(Uri uri) => uri.scheme == 'file' || uri.scheme == '';