Improves options validation type safety (#24885).

pkg/analyzer/lib/source/analysis_options_provider.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.
 
 library source.analysis_options_provider;
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/util/yaml.dart';
 import 'package:source_span/source_span.dart';
@@ skipping 36 matching lines @@
     // Empty options.
     if (doc is YamlScalar && doc.value == null) {
       return options;
     }
     if ((doc != null) && (doc is! YamlMap)) {
       throw new OptionsFormatException(
           'Bad options file format (expected map, got ${doc.runtimeType})',
           doc.span);
     }
     if (doc is YamlMap) {
-      (doc as YamlMap).forEach((k, v) {
-        if (k is! String) {
+      (doc as YamlMap).nodes.forEach((k, v) {
+        var key;
+        if (k is YamlScalar) {
+          key = k.value;
+        }
+        if (key is! String) {
           throw new OptionsFormatException(
               'Bad options file format (expected String scope key, '
               'got ${k.runtimeType})',
               k != null ? k.span : doc.span);
         }
-        options[k] = v;
+        if (v != null && v is! YamlNode) {
+          throw new OptionsFormatException(
+              'Bad options file format (expected Node value, '
+                  'got ${v.runtimeType}: `${v.toString()}`)',
+              doc.span);
+        }
+        options[key] = v;
       });
     }
     return options;
   }
 
   /// Merge the given options contents where the values in [defaults] may be
   /// overridden by [overrides].
   ///
   /// Some notes about merge semantics:
   ///
@@ skipping 23 matching lines @@
 /// Thrown on options format exceptions.
 class OptionsFormatException implements Exception {
   final String message;
   final SourceSpan span;
   OptionsFormatException(this.message, [this.span]);
 
   @override
   String toString() =>
       'OptionsFormatException: ${message?.toString()}, ${span?.toString()}';
 }