Remove deprecated Strings class.

tests/language/named_parameters_with_dollars_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.
 
 // Test to stress Frog's named parameter scheme.
 
 main() {
   testDollar();
   testPsycho();
 }
@@ skipping 10 matching lines @@
   if (thing == null) return '-';
   if (thing is List) {
     var fragments = ['['];
     var sep;
     for (final item in thing) {
       if (sep != null) fragments.add(sep);
       sep = ', ';
       fragments.add(format(item));
     }
     fragments.add(']');
-    return Strings.concatAll(fragments);
+    return fragments.join();
   }
   return thing.toString();
 }
 
 // Hopefully inscrutable to static analysis.
 makeTestClass(n) => [new TestClass(), new Decoy(), 'string'][n % 3];
 
 class Decoy {
   method([a$b, b, a]) { throw new UnimplementedError(); }
   psycho([$$$, $$, $])  { throw new UnimplementedError(); }
@@ skipping 33 matching lines @@
   Expect.equals('[1, 2, -, -]', format(t.psycho($:1, $$:2)));
   Expect.equals('[-, -, -, 4]', format(t.psycho($$$$: 4)));
 
   var obj = makeTestClass(0);
 
   Expect.equals('[1, 2, -, -]', format(obj.psycho($:1, $$:2)));
   Expect.equals('[-, -, -, 4]', format(obj.psycho($$$$: 4)));
   Expect.equals('[1, 2, 3, -]', format(obj.psycho($:1, $$:2, $$$:3)));
   Expect.equals('[1, 2, 3, -]', format(obj.psycho($$$:3, $$:2, $:1)));
 }