Move some core classes to collection library.

sdk/lib/_internal/compiler/implementation/string_validator.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.
 
 // Check the validity of string literals.
 
 library stringvalidator;
 
+import "dart:collection";
+
 import "dart2jslib.dart";
 import "tree/tree.dart";
 import "elements/elements.dart";
 import "util/characters.dart";
 import "scanner/scannerlib.dart" show Token;
 
 class StringValidator {
   final DiagnosticListener listener;
 
   StringValidator(this.listener);
@@ skipping 184 matching lines @@
       return null;
     }
     // String literal successfully validated.
     if (quoting.raw || !containsEscape) {
       // A string without escapes could just as well have been raw.
       return new DartString.rawString(string, length);
     }
     return new DartString.escapedString(string, length);
   }
 }