pkg/test: silence analyzer about downcast

test/util/string_literal_iterator_test.dart

Index: test/util/string_literal_iterator_test.dart
diff --git a/test/util/string_literal_iterator_test.dart b/test/util/string_literal_iterator_test.dart
index 0ab3104d6718ad86b6e4f1f534095185d4d10815..4b243dc798cbaa1a55dc2109c86d0179ce55b659 100644
--- a/test/util/string_literal_iterator_test.dart
+++ b/test/util/string_literal_iterator_test.dart
@@ -239,7 +239,8 @@ Matcher _isRune(String char) {
 /// Parses [dart], which should be a string literal, into a
 /// [StringLiteralIterator].
 StringLiteralIterator _parse(String dart) {
-  var literal = parseCompilationUnit("final str = $dart;")
-      .declarations.single.variables.variables.single.initializer;
+  var declaration =
+      parseCompilationUnit("final str = $dart;").declarations.single;
+  var literal = declaration.variables.variables.single.initializer;
   return new StringLiteralIterator(literal);
 }