Implement Symbol correctly in dart2js.

dart/sdk/lib/_collection_dev/symbol.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.
 
 part of dart._collection.dev;
 
 /**
  * Implementation of [core.Symbol].  This class uses the same name as
  * a core class so a user can't tell the difference.
  *
  * The purpose of this class is to hide [_name] from user code, but
  * make it accessible to Dart platform code via the static method
  * [getName].
  */
 class Symbol implements core.Symbol {
   final String _name;
 
+  static final RegExp validationPattern =
+      new RegExp(r'^(?:[a-zA-Z$][a-zA-Z$0-9_]*\.)*(?:[a-zA-Z$][a-zA-Z$0-9_]*=?|'
+                 r'-|'
+                 r'unary-|'
+                 r'\[\]=|'
+                 r'~|'
+                 r'==|'
+                 r'\[\]|'
+                 r'\*|'
+                 r'/|'
+                 r'%|'
+                 r'~/|'
+                 r'\+|'
+                 r'<<|'
+                 r'>>|'
+                 r'>=|'
+                 r'>|'
+                 r'<=|'
+                 r'<|'
+                 r'&|'
+                 r'\^|'
+                 r'\|'
+                 r')$');
+
   external const Symbol(String name);
 
   /**
    * Platform-private method used by the mirror system to create
    * otherwise invalid names.
    */
   const Symbol.unvalidated(this._name);
 
+  // This is called by dart2js.
+  Symbol.validated(String name)
+      : this._name = validate(name);
+
   bool operator ==(other) => other is Symbol && _name == other._name;
 
   int get hashCode {
     const arbitraryPrime = 664597;
     return 0x1fffffff & (arbitraryPrime * _name.hashCode);
   }
 
   /// Platform-private accessor which cannot be called from user libraries.
   static String getName(Symbol symbol) => symbol._name;
+
+  static String validate(String name) {
+    if (name.isEmpty) return name;
+    if (name.startsWith('_')) {
+      throw new ArgumentError('"$name" is a private identifier');
+    }
+    if (!validationPattern.hasMatch(name)) {
+      throw new ArgumentError(
+          '"$name" is not an identifier or an empty String');
+    }
+    return name;
+  }
 }