Fix some strong mode issues.

lib/ast.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.
 
 /// -----------------------------------------------------------------------
 ///                           ERROR HANDLING
 /// -----------------------------------------------------------------------
 ///
 /// As a rule of thumb, errors that can be detected statically are handled by
 /// the frontend, typically by translating the erroneous code into a 'throw' or
@@ skipping 1229 matching lines @@
     // This method assumes the program is correctly typed, so if the superclass
     // is not generic, we can just return its raw type without computing the
     // type of this expression.  It also ensures that all types are considered
     // subtypes of Object (not just interface types), and function types are
     // considered subtypes of Function.
     if (superclass.typeParameters.isEmpty) {
       return superclass.rawType;
     }
     var type = getStaticType(types);
     while (type is TypeParameterType) {
-      type = type.parameter.bound;
+      type = (type as TypeParameterType).parameter.bound;
     }
     if (type is InterfaceType) {
       var upcastType = types.hierarchy.getTypeAsInstanceOf(type, superclass);
       if (upcastType != null) return upcastType;
     } else if (type is BottomType) {
       return superclass.bottomType;
     }
     types.typeError(this, '$type is not a subtype of $superclass');
     return superclass.rawType;
   }
@@ skipping 2381 matching lines @@
 
   @override
   defaultTreeNode(TreeNode node) {
     if (node == child) {
       return replacement;
     } else {
       return node;
     }
   }
 }