add support for _* versions of the url-like attributes

pkg/polymer/lib/src/build/import_inliner.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 inlines polymer-element definitions from html imports.
 library polymer.src.build.import_inliner;
 
 import 'dart:async';
 import 'dart:convert';
 
@@ skipping 319 matching lines @@
     return changed;
   }
 
   visitElement(Element node) {
     // TODO(jakemac): Support custom elements that extend html elements which
     // have url-like attributes. This probably means keeping a list of which
     // html elements support each url-like attribute.
     if (!isCustomTagName(node.localName)) {
       node.attributes.forEach((name, value) {
         if (_urlAttributes.contains(name)) {
+          if (!name.startsWith('_') && value.contains(_BINDINGS)) {
+            transform.logger.warning(
+                'When using bindings with the "$name" attribute you may '
+                'experience errors in certain browsers. Please use the '
+                '"_$name" attribute instead. For more information, see '
+                'http://goo.gl/5av8cU', span: node.sourceSpan, asset: sourceId);
+          } else if (name.startsWith('_') && !value.contains(_BINDINGS)) {
+            transform.logger.warning(
+                'The "$name" attribute is only supported when using bindings. '
+                'Please change to the "${name.substring(1)}" attribute.',
+                span: node.sourceSpan, asset: sourceId);
+          }
           if (value != '' && !value.trim().startsWith(_BINDINGS)) {
             node.attributes[name] = _newUrl(value, node.sourceSpan);
             changed = changed || value != node.attributes[name];
           }
         }
       });
     }
     if (node.localName == 'style') {
       node.text = visitCss(node.text);
       changed = true;
@@ skipping 100 matching lines @@
     return builder.relative(builder.join('/', id.path),
         from: builder.join('/', builder.dirname(primaryId.path)));
   }
 }
 
 /// HTML attributes that expect a URL value.
 /// <http://dev.w3.org/html5/spec/section-index.html#attributes-1>
 ///
 /// Every one of these attributes is a URL in every context where it is used in
 /// the DOM. The comments show every DOM element where an attribute can be used.
+///
+/// The _* version of each attribute is also supported, see http://goo.gl/5av8cU
 const _urlAttributes = const [
-  'action',     // in form
-  'background', // in body
-  'cite',       // in blockquote, del, ins, q
-  'data',       // in object
-  'formaction', // in button, input
-  'href',       // in a, area, link, base, command
-  'icon',       // in command
-  'manifest',   // in html
-  'poster',     // in video
-  'src',        // in audio, embed, iframe, img, input, script, source, track,
-                //    video
+  'action', '_action',          // in form
+  'background', '_background',  // in body
+  'cite', '_cite',              // in blockquote, del, ins, q
+  'data', '_data',              // in object
+  'formaction', '_formaction',  // in button, input
+  'href', '_href',              // in a, area, link, base, command
+  'icon', '_icon',              // in command
+  'manifest', '_manifest',      // in html
+  'poster', '_poster',          // in video
+  'src', '_src',                // in audio, embed, iframe, img, input, script,
+                                //    source, track,video
 ];
 
 /// When inlining <link rel="stylesheet"> tags copy over all attributes to the
 /// style tag except these ones.
 const IGNORED_LINKED_STYLE_ATTRS =
     const ['charset', 'href', 'href-lang', 'rel', 'rev'];
 
 /// Global RegExp objects for validating generated library names.
 final INVALID_LIB_CHARS_REGEX = new RegExp('[^a-z0-9_]');
 final NUM_REGEX = new RegExp('[0-9]');
 
 _getSpan(SourceFile file, AstNode node) => file.span(node.offset, node.end);