[polymer] interop with polymer-element and polymer.js

pkg/polymer/lib/src/js/use_native_dartium_shadowdom.js

Index: pkg/polymer/lib/src/js/use_native_dartium_shadowdom.js
diff --git a/pkg/polymer/lib/src/js/use_native_dartium_shadowdom.js b/pkg/polymer/lib/src/js/use_native_dartium_shadowdom.js
new file mode 100644
index 0000000000000000000000000000000000000000..c2f51c1a60584267105f16d7f87e8b407273fb12
--- /dev/null
+++ b/pkg/polymer/lib/src/js/use_native_dartium_shadowdom.js
@@ -0,0 +1,10 @@
+// Prevent polyfilled JS Shadow DOM in Dartium
+// We need this if we want Dart code to be able to interoperate with Polymer.js
+// code that also uses Shadow DOM.
+// TODO(jmesserly): we can remove this code once platform.js is correctly
+// feature detecting Shadow DOM in Dartium.
+if (navigator.userAgent.indexOf('(Dart)') !== -1) {
+  window.Platform = window.Platform || {};
+  Platform.flags = Platform.flags || {};
+  Platform.flags.shadow = 'native';
+}