fix #569, cache constants defined in methods

lib/src/compiler/module_builder.dart

 // Copyright (c) 2015, 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.
 
 import 'package:path/path.dart' as path;
 
 import '../js_ast/js_ast.dart';
 import 'js_names.dart';
 
 /// Base class for compiling ES6 modules into various ES5 module patterns.
@@ skipping 132 matching lines @@
     // This extra level of indirection should be unnecessary.
     var block =
         js.statement("(function() { 'use strict'; #; })()", [statements]);
 
     return new Program([block]);
   }
 }
 
 /// Escape [name] to make it into a valid identifier.
 String pathToJSIdentifier(String name) {
-  name = path.basenameWithoutExtension(name);
+  return toJSIdentifier(path.basenameWithoutExtension(name));
+}
+
+/// Escape [name] to make it into a valid identifier.
+String toJSIdentifier(String name) {
   if (name.length == 0) return r'$';
 
   // Escape any invalid characters
   StringBuffer buffer = null;
   for (int i = 0; i < name.length; i++) {
     var ch = name[i];
     var needsEscape = ch == r'$' || _invalidCharInIdentifier.hasMatch(ch);
     if (needsEscape && buffer == null) {
       buffer = new StringBuffer(name.substring(0, i));
     }
     if (buffer != null) {
       buffer.write(needsEscape ? '\$${ch.codeUnits.join("")}' : ch);
     }
   }
 
   var result = buffer != null ? '$buffer' : name;
   // Ensure the identifier first character is not numeric and that the whole
   // identifier is not a keyword.
   if (result.startsWith(new RegExp('[0-9]')) || invalidVariableName(result)) {
     return '\$$result';
   }
   return result;
 }
 
 // Invalid characters for identifiers, which would need to be escaped.
 final _invalidCharInIdentifier = new RegExp(r'[^A-Za-z_$0-9]');