Associate excludes with context data.

pkg/analyzer/lib/src/util/yaml.dart

Index: pkg/analyzer/lib/src/util/yaml.dart
diff --git a/pkg/analyzer/lib/src/util/yaml.dart b/pkg/analyzer/lib/src/util/yaml.dart
index 4f71bd74869f7ec6c6bdddc66cea11dd46d32112..f53086e44c881fc4d3c5aa7178c9c33abc1e54cd 100644
--- a/pkg/analyzer/lib/src/util/yaml.dart
+++ b/pkg/analyzer/lib/src/util/yaml.dart
@@ -6,6 +6,25 @@ library analyzer.src.util.yaml;
 
 import 'dart:collection';
 
+import 'package:yaml/yaml.dart';
+
+/// If all of the elements of [list] are strings, return a list of strings
+/// containing the same elements. Otherwise, return `null`.
+List<String> toStringList(YamlList list) {
+  if (list == null) {
+    return null;
+  }
+  List<String> stringList = <String>[];
+  for (var element in list) {
+    if (element is String) {
+      stringList.add(element);
+    } else {
+      return null;
+    }
+  }
+  return stringList;
+}
+
 /// Merges two maps (of yaml) with simple override semantics, suitable for
 /// merging two maps where one map defines default values that are added to
 /// (and possibly overriden) by an overriding map.