simplify function coercions -- DDC can generate function types in place

lib/runtime/dart/indexed_db.js

 dart_library.library('dart/indexed_db', null, /* Imports */[
   'dart/_runtime',
   'dart/core',
   'dart/html_common',
   'dart/_js_helper',
   'dart/async',
   'dart/_interceptors',
   'dart/_metadata',
   'dart/html'
 ], /* Lazy imports */[
@@ skipping 364 matching lines @@
         let stacktrace = dart.stackTrace(e);
         return async.Future$(Database).error(e, stacktrace);
       }
 
     }
     [dartx.deleteDatabase](name, opts) {
       let onBlocked = opts && 'onBlocked' in opts ? opts.onBlocked : null;
       try {
         let request = this[_deleteDatabase](name);
         if (onBlocked != null) {
-          request[dartx.onBlocked].listen(dart.as(onBlocked, __CastType0));
+          request[dartx.onBlocked].listen(dart.as(onBlocked, dart.functionType(dart.void, [html.Event])));
         }
         let completer = async.Completer.sync();
         request[dartx.onSuccess].listen(dart.fn(e => {
           completer.complete(this);
         }, dart.void, [html.Event]));
         request[dartx.onError].listen(dart.bind(completer, 'completeError'));
         return dart.as(completer.future, async.Future$(IdbFactory));
       } catch (e) {
         let stacktrace = dart.stackTrace(e);
         return async.Future$(IdbFactory).error(e, stacktrace);
@@ skipping 708 matching lines @@
     }
     get [dartx.oldVersion]() {
       return this.oldVersion;
     }
   }
   dart.setSignature(VersionChangeEvent, {
     constructors: () => ({_: [VersionChangeEvent, []]})
   });
   VersionChangeEvent[dart.metadata] = () => [dart.const(new _metadata.DocsEditable()), dart.const(new _metadata.DomName('IDBVersionChangeEvent')), dart.const(new _metadata.Unstable()), dart.const(new _js_helper.Native("IDBVersionChangeEvent"))];
   dart.registerExtension(dart.global.IDBVersionChangeEvent, VersionChangeEvent);
-  const __CastType0 = dart.typedef('__CastType0', () => dart.functionType(dart.void, [html.Event]));
   // Exports:
   exports.Cursor = Cursor;
   exports.CursorWithValue = CursorWithValue;
   exports.Database = Database;
   exports.IdbFactory = IdbFactory;
   exports.Index = Index;
   exports.KeyRange = KeyRange;
   exports.ObjectStore = ObjectStore;
   exports.Request = Request;
   exports.OpenDBRequest = OpenDBRequest;
   exports.Transaction = Transaction;
   exports.VersionChangeEvent = VersionChangeEvent;
 });