Move absolute url logic to element-mixin

This is backwards-compatible with the previous URL resolution.
Now, `Polymer.ResolveUrl.resolveUrl` correctly appends the url
to the basePath.

lib/mixins/element-mixin.html

@@ -21,6 +21,8 @@
 (function() {
   'use strict';
 
+  const ABS_URL = /(^\/)|(^#)|(^[\w-\d]*:)/;
+
   /**
    * Element class mixin that provides the core API for Polymer's meta-programming
    * features including template stamping, data-binding, attribute deserialization,
@@ -636,6 +638,9 @@
        * @return {string} Rewritten URL relative to base
        */
       resolveUrl(url, base) {
+        if (url && ABS_URL.test(url)) {
+          return url;
+        }
         if (!base && this.importPath) {
           base = Polymer.ResolveUrl.resolveUrl(this.importPath);
         }

lib/utils/resolve-url.html

@@ -15,7 +15,6 @@
     'use strict';
 
     let CSS_URL_RX = /(url\()([^)]*)(\))/g;
-    let ABS_URL = /(^\/)|(^#)|(^[\w-\d]*:)/;
     let workingURL;
     let resolveDoc;
     /**
@@ -27,9 +26,6 @@
      * @return {string} resolved URL
      */
     function resolveUrl(url, baseURI) {
-      if (url && ABS_URL.test(url)) {
-        return url;
-      }
       // Lazy feature detection.
       if (workingURL === undefined) {
         workingURL = false;

test/unit/resolveurl.html

@@ -157,6 +157,12 @@
         assert.equal(actual, expected);
       });
     });
+
+    suite('Polymer.ResolveUrl', function() {
+      test('appends absolute url to basePath', function() {
+        assert.equal(Polymer.ResolveUrl.resolveUrl('/foo', 'http://localhost'), 'http://localhost/foo');
+      });
+    });
   </script>
 </body>
 </html>