fix #569, cache constants defined in methods

lib/src/compiler/module_builder.dart

Index: lib/src/compiler/module_builder.dart
diff --git a/lib/src/compiler/module_builder.dart b/lib/src/compiler/module_builder.dart
index 0b2be9159c0d21123dffe4184d179ea5b4f44e84..387f47a6539a30b7b1d439943bfddf3ff3f9b8dd 100644
--- a/lib/src/compiler/module_builder.dart
+++ b/lib/src/compiler/module_builder.dart
@@ -150,7 +150,11 @@ class NodeModuleBuilder extends _ModuleBuilder {
 
 /// 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