Remove deprecated Strings class.

tests/language/string_test.dart

 // Copyright (c) 2011, 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.
 // Replace with shared test once interface issues clarified.
 
 class StringTest {
 
   static testMain() {
     testCodePoints();
     testNoSuchMethod();
     testStringsJoin();
     testCharCodes();
   }
 
   static testCodePoints() {
     String str = "string";
     for (int i = 0; i < str.length; i++) {
       Expect.equals(true, str[i] is String);
       Expect.equals(true, str.charCodeAt(i) is int);
     }
   }
 
   static testStringsJoin() {
     List<String> a = new List<String>.fixedLength(2);
     a[0] = "Hello";
     a[1] = "World";
-    String s = Strings.join(a, "*^*");
+    String s = a.join("*^*");
     Expect.equals("Hello*^*World", s);
   }
 
   static testNoSuchMethod() {
     String a = "Hello";
     bool exception_caught = false;
     try {
       a[1] = 12;  // Throw exception.
     } on NoSuchMethodError catch (e) {
       exception_caught = true;
     }
     Expect.equals(true, exception_caught);
   }
 
   static testCharCodes() {
     String s = new String.fromCharCodes(const [0x41, 0xC1, 0x424]);
     Expect.equals("A", s[0]);
     Expect.equals(0x424, s.charCodeAt(2));
   }
 }
 
 main() {
   StringTest.testMain();
 }