Warn about accessing private properties, but don't choke on it.

pkg/polymer/lib/src/build/script_compactor.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.
 
 /// Transfomer that combines multiple dart script tags into a single one.
 library polymer.src.build.script_compactor;
 
 import 'dart:async';
 import 'dart:convert';
 
 import 'package:html5lib/dom.dart' show Document, Element, Text;
 import 'package:html5lib/dom_parsing.dart';
 import 'package:html5lib/parser.dart' show parseFragment;
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart' hide Element;
 import 'package:analyzer/src/generated/element.dart' as analyzer show Element;
 import 'package:barback/barback.dart';
 import 'package:path/path.dart' as path;
-import 'package:source_maps/span.dart' show SourceFile;
+import 'package:source_maps/span.dart' show SourceFile, Span;
 import 'package:smoke/codegen/generator.dart';
 import 'package:smoke/codegen/recorder.dart';
 import 'package:code_transformers/resolver.dart';
 import 'package:code_transformers/src/dart_sdk.dart';
 import 'package:template_binding/src/mustache_tokens.dart' show MustacheTokens;
 
 import 'package:polymer_expressions/expression.dart' as pe;
 import 'package:polymer_expressions/parser.dart' as pe;
 import 'package:polymer_expressions/visitor.dart' as pe;
 
@@ skipping 453 matching lines @@
 ///     will be needed to evaluate them at runtime.
 ///   * extracts all attributes declared in the `attribute` attributes of
 ///     polymer elements.
 class _HtmlExtractor extends TreeVisitor {
   final Map<String, List<String>> publishedAttributes;
   final SmokeCodeGenerator generator;
   final _SubExpressionVisitor visitor;
   final TransformLogger logger;
   bool _inTemplate = false;
 
-  _HtmlExtractor(this.logger, SmokeCodeGenerator generator,
+  _HtmlExtractor(TransformLogger logger, SmokeCodeGenerator generator,
       this.publishedAttributes)
-      : generator = generator,
-        visitor = new _SubExpressionVisitor(generator);
+      : logger = logger, generator = generator,
+        visitor = new _SubExpressionVisitor(generator, logger);
 
   void visitElement(Element node) {
     if (_inTemplate) _processNormalElement(node);
     if (node.localName == 'polymer-element') {
       _processPolymerElement(node);
       _processNormalElement(node);
     }
 
     if (node.localName == 'template') {
       var last = _inTemplate;
@@ skipping 44 matching lines @@
             tag == 'select' && (name == 'selectedindex' || name == 'value') ||
             tag == 'textarea' && name == 'value');
       }
       for (var exp in bindings.expressions) {
         _addExpression(exp, isEvent, isTwoWay, node.sourceSpan);
       }
     });
   }
 
   void _addExpression(String stringExpression, bool inEvent, bool isTwoWay,
-      span) {
+      Span span) {
 
     if (inEvent) {
       if (stringExpression.startsWith('@')) {
         logger.warning('event bindings with @ are no longer supported',
             span: span);
         return;
       }
 
       if (stringExpression == '') return;
+      if (stringExpression.startsWith('_')) {
+        logger.warning('private symbols cannot be used in event handlers',
+            span: span);
+        return;
+      }
       generator.addGetter(stringExpression);
       generator.addSymbol(stringExpression);
     }
-    visitor.run(pe.parse(stringExpression), isTwoWay);
+    visitor.run(pe.parse(stringExpression), isTwoWay, span);
   }
 }
 
 /// A polymer-expression visitor that records every getter and setter that will
 /// be needed to evaluate a single expression at runtime.
 class _SubExpressionVisitor extends pe.RecursiveVisitor {
   final SmokeCodeGenerator generator;
+  final TransformLogger logger;
   bool _includeSetter;
+  Span _currentSpan;
 
-  _SubExpressionVisitor(this.generator);
+  _SubExpressionVisitor(this.generator, this.logger);
 
   /// Visit [exp], and record getters and setters that are needed in order to
   /// evaluate it at runtime. [includeSetter] is only true if this expression
   /// occured in a context where it could be updated, for example in two-way
   /// bindings such as `<input value={{exp}}>`.
-  void run(pe.Expression exp, bool includeSetter) {
+  void run(pe.Expression exp, bool includeSetter, span) {
+    _currentSpan = span;
     _includeSetter = includeSetter;
     visit(exp);
   }
 
   /// Adds a getter and symbol for [name], and optionally a setter.
   _add(String name) {
+    if (name.startsWith('_')) {
+      logger.warning('private symbols are not supported', span: _currentSpan);
+      return;
+    }
     generator.addGetter(name);

[Siggi Cherem (dart-lang), 2014/06/24 00:58:13]

these APIs will cause a later failure if they are given private symbols.

     generator.addSymbol(name);
     if (_includeSetter) generator.addSetter(name);
   }
 
   void preVisitExpression(e) {
     // For two-way bindings the outermost expression may be updated, so we need
     // both the getter and the setter, but we only need the getter for
     // subexpressions. We exclude setters as soon as we go deeper in the tree,
     // except when we see a filter (that can potentially be a two-way
     // transformer).
@@ skipping 162 matching lines @@
   for (var c in combinators) {
     if (c is ShowElementCombinator) {
       var show = c.shownNames.toSet();
       elements.retainWhere((e) => show.contains(e.displayName));
     } else if (c is HideElementCombinator) {
       var hide = c.hiddenNames.toSet();
       elements.removeWhere((e) => hide.contains(e.displayName));
     }
   }
 }