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

pkg/polymer/lib/src/build/import_inliner.dart

Index: pkg/polymer/lib/src/build/import_inliner.dart
diff --git a/pkg/polymer/lib/src/build/import_inliner.dart b/pkg/polymer/lib/src/build/import_inliner.dart
index 675db2999d793603aebda86e8799a5b407a7ecd9..c2403aab5b0ed4f29643e4e650e16da5d5d66211 100644
--- a/pkg/polymer/lib/src/build/import_inliner.dart
+++ b/pkg/polymer/lib/src/build/import_inliner.dart
@@ -337,6 +337,19 @@ class _UrlNormalizer extends TreeVisitor {
     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://www.polymer-project.org/docs/polymer/databinding-compat'

[Siggi Cherem (dart-lang), 2014/07/31 21:27:45]

Here is the URL shortener that we use in dart2js:

   http://goo.gl/

That should help here :)

[jakemac, 2014/08/01 14:24:31]

Done.

+                '.html#binding-to-attributes', 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 attribute.');
+          }
           if (value != '' && !value.trim().startsWith(_BINDINGS)) {
             node.attributes[name] = _newUrl(value, node.sourceSpan);
             changed = changed || value != node.attributes[name];
@@ -457,18 +470,21 @@ class _UrlNormalizer extends TreeVisitor {
 ///
 /// 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://www.polymer-project.org/docs/polymer/databinding-compat.html#binding-to-attributes
 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