Merge pull request #4850 from Polymer/1.x-html-imports-styling-workaround

Copy <style is="custom-style"> to the main document

src/lib/custom-style.html

@@ -123,6 +123,19 @@ Note, all features of `custom-style` are available when defining styles as part
       if (this.__appliedElement !== this) {
         this.__appliedElement.__cssBuild = this.__cssBuild;
       }
+      /*
+      HTML Imports styling the main document are deprecated in Chrome
+      https://crbug.com/523952
+
+      If this element is not in the main document, then it must be in an HTML Import document.
+      In that case, move the custom style to the main document.
+
+      The ordering of `<custom-style>` should stay the same as when loaded by HTML Imports, but there may be odd
+      cases of ordering w.r.t the main document styles.
+      */
+      if (this.ownerDocument !== window.document && this.__appliedElement === this) {
+        document.head.appendChild(this);
+      }
       // needed becuase elements in imports do not get 'attached'
       // TODO(sorvell): we could only do this if and only if
       // this.ownerDocument != document;