pkg/test: silence analyzer about downcast

test/util/string_literal_iterator_test.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.
 
 @TestOn("vm")
 
 import 'package:analyzer/analyzer.dart';
 import 'package:test/test.dart';
 import 'package:test/src/util/string_literal_iterator.dart';
 
@@ skipping 221 matching lines @@
 /// Returns a matcher that asserts that the given rune is the rune for [char].
 Matcher _isRune(String char) {
   return predicate((rune) {
     return rune is int && new String.fromCharCode(rune) == char;
   }, 'is the rune "$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);
 }