fixes #610, incorrect help output

lib/src/compiler/element_helpers.dart

 // Copyright (c) 2016, 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.
 
 /// Helpers for Analyzer's Element model and corelib model.
 
 import 'package:analyzer/dart/ast/ast.dart'
     show
         ConstructorDeclaration,
         Expression,
@@ skipping 11 matching lines @@
     show DartObject, DartObjectImpl;
 
 class Tuple2<T0, T1> {
   final T0 e0;
   final T1 e1;
   Tuple2(this.e0, this.e1);
 }
 
 /*=T*/ fillDynamicTypeArgs/*<T extends DartType>*/(/*=T*/ t) {
   if (t is ParameterizedType) {
-    var pt = t as ParameterizedType;
     var dyn = new List<DartType>.filled(
-        pt.typeArguments.length, DynamicTypeImpl.instance);
-    return pt.substitute2(dyn, pt.typeArguments) as dynamic/*=T*/;
+        t.typeArguments.length, DynamicTypeImpl.instance);
+    return t.substitute2(dyn, t.typeArguments) as dynamic/*=T*/;
   }
   return t;
 }
 
 /// Given an annotated [node] and a [test] function, returns the first matching
 /// constant valued annotation.
 ///
 /// For example if we had the ClassDeclaration node for `FontElement`:
 ///
 ///    @js.JS('HTMLFontElement')
@@ skipping 68 matching lines @@
 ///       const MyAnnotation(this.name/*, ... other params ... */);
 ///     }
 ///
 ///     @MyAnnotation('FooBar')
 ///     main() { ... }
 ///
 /// If we match the annotation for the `@MyAnnotation('FooBar')` this will
 /// return the string 'FooBar'.
 String getAnnotationName(Element element, bool match(DartObjectImpl value)) =>
     findAnnotation(element, match)?.getField('name')?.toStringValue();