Remove deprecated Strings class.

tools/testing/dart/multitest.dart

 // Copyright (c) 2012, 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 multitest;
 
 import "dart:async";
 import "dart:io";
 import "test_suite.dart";
 import "utils.dart";
@@ skipping 104 matching lines @@
                   "Expected one of: ${validMultitestOutcomes.toString()}");
     }
   }
 
   // Add the template, with no multitest lines, as a test with key 'none'.
   testsAsLines['none'] = testTemplate;
   outcomes['none'] = new Set<String>();
 
   // Copy all the tests into the output map tests, as multiline strings.
   for (String key in testsAsLines.keys) {
-    tests[key] =
-        Strings.join(testsAsLines[key], line_separator).concat(line_separator);
+    tests[key] = testsAsLines[key].join(line_separator).concat(line_separator);
   }
 }
 
 // Represents a mutlitest annotation in the special /// comment.
 class _Annotation {
   String key;
   String rest;
   List<String> outcomesList;
   _Annotation() {}
   factory _Annotation.from(String line) {
@@ skipping 119 matching lines @@
     // TestSuite.forDirectory.
     split.removeLast();
   }
   String path = '${generatedTestDir.path}/${split.last}';
   Directory dir = new Directory(path);
   if (!dir.existsSync()) {
     dir.createSync();
   }
   return new Path(new File(path).fullPathSync());
 }