Change how we check for user agent, in preparation for breaking change in Dartium.

pkg/polymer/lib/boot.js

Index: pkg/polymer/lib/boot.js
diff --git a/pkg/polymer/lib/boot.js b/pkg/polymer/lib/boot.js
index 62d72a7cac5ea429a3d5ae9a6b4fab193d518cdb..419ebb12ea8a2e67c60addbbce8a053f5c327bd5 100644
--- a/pkg/polymer/lib/boot.js
+++ b/pkg/polymer/lib/boot.js
@@ -33,7 +33,12 @@
 /// discovered in the HTML document.
 (function() {
   // Only run in Dartium.
-  if (navigator.userAgent.indexOf('(Dart)') === -1) return;
+  if (!navigator.dartEnabled &&
+      // TODO(sigmund): remove userAgent check once 1.6 rolls as stable.
+      // See: dartbug.com/18463
+      (navigator.userAgent.indexOf('(Dart)') === -1)) {
+    return;
+  }
 
   // Extract a Dart import URL from a script tag, which is the 'src' attribute
   // of the script tag, or a data-url with the script contents for inlined code.