Fix all strong-mode errors and warnings.

lib/src/runner/browser/polymer.dart

 // Copyright (c) 2015, 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:io';
 
+import 'package:collection/collection.dart';
 import 'package:glob/glob.dart';
 import 'package:path/path.dart' as p;
 import 'package:yaml/yaml.dart';
 
-/// Whether the package being tested uses the Polymer transforemrs at all.
+/// Whether the package being tested uses the Polymer transformers at all.
 ///
 /// This will be `null` until [_initialize] is called.
 bool _usesPolymer;
 
 /// The set of entry points passed in to the polymer transformer's config.
 ///
 /// This will be `null` if no entrypoints are declared, indicating that all
 /// entrypoints are supported.
 Set<String> _entrypoints;
 
@@ skipping 51 matching lines @@
         if (transformer != "polymer") continue;
         _usesPolymer = true;
         return;
       }
 
       if (transformer.keys.single != "polymer") continue;
       _usesPolymer = true;
 
       var configuration = transformer.values.single;
 
-      var entrypoints = configuration["entry_points"];
-      if (entrypoints != null) _entrypoints = entrypoints.toSet();
+      var entrypoints = configuration["entry_points"] as List;
+      if (entrypoints != null) {
+        _entrypoints = DelegatingSet.typed(entrypoints.toSet());
+      }
 
       _includes = _parseGlobField(configuration, r"$includes");
       _excludes = _parseGlobField(configuration, r"$excludes");
       return;
     }
   }
 }
 
 /// Parses a glob field (either `$include` or `$exclude`).
 Set<Glob> _parseGlobField(YamlMap configuration, String name) {
   if (!configuration.containsKey(name)) return null;
   var field = configuration[name];
 
   if (field is String) {
     return new Set.from([new Glob(field, context: p.url, recursive: true)]);
   }
 
   return new Set.from(field.map((node) {
     return new Glob(node, context: p.url, recursive: true);
   }));
 }