Fix all strong-mode warnings.

test/yaml_test.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:test/test.dart';
 import 'package:yaml/yaml.dart';
 
 import 'utils.dart';
 
 main() {
@@ skipping 39 matching lines @@
          --- text
          """);
     });
   });
 
   test("includes source span information", () {
     var yaml = loadYamlNode(r"""
 - foo:
     bar
 - 123
-""");
+""") as YamlList;
 
     expect(yaml.span.start.line, equals(0));
     expect(yaml.span.start.column, equals(0));
     expect(yaml.span.end.line, equals(3));
     expect(yaml.span.end.column, equals(0));
 
-    var map = yaml.nodes.first;
+    var map = yaml.nodes.first as YamlMap;
     expect(map.span.start.line, equals(0));
     expect(map.span.start.column, equals(2));
     expect(map.span.end.line, equals(2));
     expect(map.span.end.column, equals(0));
 
     var key = map.nodes.keys.first;
     expect(key.span.start.line, equals(0));
     expect(key.span.start.column, equals(2));
     expect(key.span.end.line, equals(0));
     expect(key.span.end.column, equals(5));
@@ skipping 1796 matching lines @@
         A null: null
         Also a null: # Empty
         Not a null: ""
         Booleans: [ true, True, false, FALSE ]
         Integers: [ 0, 0o7, 0x3A, -19 ]
         Floats: [ 0., -0.0, .5, +12e03, -2E+05 ]
         Also floats: [ .inf, -.Inf, +.INF, .NAN ]''');
     });
   });
 }