Bring the rest of the polymer.dart code from github.

pkg/polymer/lib/polymer.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * This library exports all of the commonly used functions and types for
  * building UI's.
  *
  * See this article for more information:
  * <http://www.dartlang.org/articles/dart-web-components/>.
@@ skipping 21 matching lines @@
 /** Annotation used to automatically register polymer elements. */
 class CustomTag {
   final String tagName;
   const CustomTag(this.tagName);
 }
 
 /**
  * Metadata used to label static or top-level methods that are called
  * automatically when loading the library of a custom element.
  */
-const polymerInitMethod = const _InitPolymerAnnotation();
+const initMethod = const _InitMethodAnnotation();
 
 /**
- * Initializes a polymer application by: setting up polling for observable
- * changes, initializing MDV, registering and initializing custom elements from
- * each library in [elementLibraries], and finally invoking [userMain].
+ * Initializes a polymer application as follows:
+ *   * set up up polling for observable changes
+ *   *  initialize MDV
+ *   *  for each library in [libraries], register custom elements labeled with
+ *      [CustomTag] and invoke the initialization method on it.
  *
- * There are two mechanisms by which custom elements can be initialized:
- * annotating the class that declares a custom element with [CustomTag] or
- * programatically registering the element in a static or top-level function and
- * annotating that function with [polymerInitMethod].
+ * The initialization on each library is either a method named `main` or
+ * a top-level function and annotated with [initMethod].
  *
- * The urls in [elementLibraries] can be absolute or relative to [srcUrl].
+ * The urls in [libraries] can be absolute or relative to [srcUrl].
  */
-void initPolymer(List<String> elementLibraries, void userMain(), [String srcUrl]) {
+void initPolymer(List<String> libraries, [String srcUrl]) {
   wrapMicrotask(() {
     // DOM events don't yet go through microtasks, so we catch those here.
     new Timer.periodic(new Duration(milliseconds: 125),
         (_) => performMicrotaskCheckpoint());
 
     // TODO(jmesserly): mdv should use initMdv instead of mdv.initialize.
     mdv.initialize();
-    for (var lib in elementLibraries) {
-      _registerPolymerElementsOf(lib, srcUrl);
+    for (var lib in libraries) {
+      _loadLibrary(lib, srcUrl);
     }
-    userMain();
   })();
 }
 
 /** All libraries in the current isolate. */
 final _libs = currentMirrorSystem().libraries;
 
 /**
  * Reads the library at [uriString] (which can be an absolute URI or a relative
  * URI from [srcUrl]), and:
  *
- *   * Invokes top-level and static functions marked with the
- *     [polymerInitMethod] annotation.
+ *   * If present, invokes `main`.
+ *
+ *   * If present, invokes any top-level and static functions marked
+ *     with the [initMethod] annotation (in the order they appear).
  *
  *   * Registers any [PolymerElement] that is marked with the [CustomTag]
  *     annotation.
  */
-void _registerPolymerElementsOf(String uriString, [String srcUrl]) {
+void _loadLibrary(String uriString, [String srcUrl]) {
   var uri = Uri.parse(uriString);
   if (uri.scheme == '' && srcUrl != null) {
     uri = Uri.parse(path.normalize(path.join(path.dirname(srcUrl), uriString)));
   }
   var lib = _libs[uri];
   if (lib == null) {
     print('warning: $uri library not found');
     return;
   }
 
-  // Search top-level functions marked with @polymerInitMethod
+  // Invoke `main`, if present.
+  if (lib.functions[const Symbol('main')] != null) {
+    lib.invoke(const Symbol('main'), const []);
+  }
+
+  // Search top-level functions marked with @initMethod
   for (var f in lib.functions.values) {
     _maybeInvoke(lib, f);
   }
 
   for (var c in lib.classes.values) {
     // Search for @CustomTag on classes
     for (var m in c.metadata) {
       var meta = m.reflectee;
       if (meta is CustomTag) {
         registerPolymerElement(meta.tagName,
             () => c.newInstance(const Symbol(''), const []).reflectee);
       }
     }
 
-    // TODO(sigmund): check also static methods marked with @polymerInitMethod.
+    // TODO(sigmund): check also static methods marked with @initMethod.
     // This is blocked on two bugs:
     //  - dartbug.com/12133 (static methods are incorrectly listed as top-level
     //    in dart2js, so they end up being called twice)
     //  - dartbug.com/12134 (sometimes "method.metadata" throws an exception,
     //    we could wrap and hide those exceptions, but it's not ideal).
   }
 }
 
 void _maybeInvoke(ObjectMirror obj, MethodMirror method) {
   var annotationFound = false;
   for (var meta in method.metadata) {
-    if (identical(meta.reflectee, polymerInitMethod)) {
+    if (identical(meta.reflectee, initMethod)) {
       annotationFound = true;
       break;
     }
   }
   if (!annotationFound) return;
   if (!method.isStatic) {
-    print("warning: methods marked with @polymerInitMethod should be static,"
+    print("warning: methods marked with @initMethod should be static,"
         " ${method.simpleName} is not.");
     return;
   }
   if (!method.parameters.where((p) => !p.isOptional).isEmpty) {
-    print("warning: methods marked with @polymerInitMethod should take no "
+    print("warning: methods marked with @initMethod should take no "
         "arguments, ${method.simpleName} expects some.");
     return;
   }
   obj.invoke(method.simpleName, const []);
 }
 
-class _InitPolymerAnnotation {
-  const _InitPolymerAnnotation();
+class _InitMethodAnnotation {
+  const _InitMethodAnnotation();
 }