Have boot create Polymer() fn, and polymer-fn install _polymerFn callback.

polymer.html

@@ -1,6 +1,5 @@
 <link rel="import" href="src/legacy/legacy-element-mixin.html">
 <link rel="import" href="src/legacy/polymer-fn.html">
-<link rel="import" href="src/legacy/class.html">
 <!-- template elements -->
 <link rel="import" href="src/legacy/templatizer-behavior.html">
 <link rel="import" href="src/elements/dom-bind.html">

src/legacy/polymer-fn.html

@@ -7,20 +7,14 @@ The complete set of contributors may be found at http://polymer.github.io/CONTRI
 Code distributed by Google as part of the polymer project is also
 subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
 -->
+
+<link rel="import" href="class.html">
+
 <script>
 
   (function() {
 
-    // TODO(sorvell): There is an implicit depency on `Polymer.Class` here
-    // that would be good to resolve. It's because (elements/class.html)
-    // because this file specially handles `Polymer`.
-
-    // Polymer is a Function, but of course this is also an Object, so we
-    // hang various other objects off of Polymer.*
-
-    var userPolymer = window['Polymer'];
-
-    window['Polymer'] = function(info) {
+    window.Polymer._polymerFn = function(info) {
       // if input is a `class` (aka a function with a prototype), use the prototype
       // remember that the `constructor` will never be called
       var klass;
@@ -37,12 +31,6 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
       return klass;
     };
 
-    if (userPolymer) {
-      for (var i in userPolymer) {
-        Polymer[i] = userPolymer[i];
-      }
-    }
-
   })();
 
 </script>

src/lib/boot.html

@@ -9,7 +9,13 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
 -->
 <script>
 
-  window.Polymer = window.Polymer || {};
+  window.Polymer = function(info) {
+    return window.Polymer._polymerFn(info);
+  }
+  // To be plugged by legacy implementation if loaded
+  window.Polymer._polymerFn = function() {
+    throw new Error('Load polymer.html to use the Polymer() function.');
+  }
   window.Polymer.version = '2.0-preview';
 
   /* eslint-disable no-unused-vars */