Remove deprecated Strings class.

pkg/scheduled_test/test/metatest.dart

 // Copyright (c) 2013, 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.
 
 /// A test library for testing test libraries? We must go deeper.
 ///
 /// Since unit testing code tends to use a lot of global state, it can be tough
 /// to test. This library manages it by running each test case in a child
 /// isolate, then reporting the results back to the parent isolate.
 library metatest;
@@ skipping 152 matching lines @@
         '${results['errors']} ERRORS');
   }
   return prefixLines(buffer.toString());
 }
 
 /// Indents each line of [str] by two spaces.
 String _indent(String str) {
   // TODO(nweiz): Use this simpler code once issue 2980 is fixed.
   // return str.replaceAll(new RegExp("^", multiLine: true), "  ");
 
-  return Strings.join(str.split("\n").map((line) => "  $line"), "\n");
+  return str.split("\n").map((line) => "  $line").join("\n");
 }
 
 /// Ensure that the metatest configuration is loaded.
 void _ensureInitialized() {
   if (config is! _MetaConfiguration) configure(new _MetaConfiguration());
 }
 
 /// Special test configuration for use within the child isolates. This hides all
 /// output and reports data back to the parent isolate.
 class _MetaConfiguration extends Configuration {
@@ skipping 13 matching lines @@
         "message": testCase.message,
         "result": testCase.result,
         "stackTrace": testCase.stackTrace
       }).toList()
     });
   }
 
   void onInit() {}
   void onDone(bool success) {}
 }