Fix 19029: handle special case when on-handler is empty in polymer, add better

pkg/polymer/lib/src/build/linter.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.
 
 /// Logic to validate that developers are correctly using Polymer constructs.
 /// This is mainly used to produce warnings for feedback in the editor.
 library polymer.src.build.linter;
 
 import 'dart:async';
 
@@ skipping 349 matching lines @@
           'dashes such as "$name". Use "$newName" or "${newName.toLowerCase()}" '
           'instead (both forms are equivalent in HTML).', span: span);
       return false;
     }
     return true;
   }
 
   /// Validate event handlers are used correctly.
   void _validateEventHandler(Element node, String name, String value) {
     if (!name.startsWith('on-')) {
+      // TODO(sigmund): technically these are valid attribtues in HTML, so we
+      // might want to remove this warning, or only produce it if the value
+      // looks like a binding.
       _logger.warning('Event handler "$name" will be interpreted as an inline'
           ' JavaScript event handler. Use the form '
-          'on-event-name="handlerName" if you want a Dart handler '
+          'on-event-name="{{handlerName}}" if you want a Dart handler '
           'that will automatically update the UI based on model changes.',
           span: node.attributeSpans[name]);
       return;
     }
 
     if (!_inPolymerElement) {
       _logger.warning('Inline event handlers are only supported inside '
           'declarations of <polymer-element>.',
           span: node.attributeSpans[name]);
+      return;
     }
 
-    if (value.contains('(')) {
+
+    // Valid bindings have {{ }}, don't look like method calls foo(bar), and are
+    // non empty.
+    if (!value.startsWith("{{") || !value.endsWith("}}") || value.contains('(')
+        || value.substring(2, value.length - 2).trim() == '') {
       _logger.warning('Invalid event handler body "$value". Declare a method '
           'in your custom element "void handlerName(event, detail, target)" '
-          'and use the form $name="handlerName".',
+          'and use the form $name="{{handlerName}}".',
           span: node.attributeSpans[name]);
     }
   }
 }
 
 const String ONLY_ONE_TAG =
     'Only one "application/dart" script tag per document is allowed.';
 
 const String USE_POLYMER_HTML =
     'Besides the initPolymer invocation, to run a polymer application you need '
     'to include the following HTML import: '
     '<link rel="import" href="packages/polymer/polymer.html">. This will '
     'include the common polymer logic needed to boostrap your application.';
 
 const String USE_INIT_DART =
     'To run a polymer application, you need to call "initPolymer". You can '
     'either include a generic script tag that does this for you:'
     '\'<script type="application/dart">export "package:polymer/init.dart";'
     '</script>\' or add your own script tag and call that function. '
     'Make sure the script tag is placed after all HTML imports.';
 
 const String NO_DART_SCRIPT_AND_EXPERIMENTAL =
     'The experimental bootstrap feature doesn\'t support script tags on '
     'the main document (for now).';