Adds typechecking of return values

frog/type.dart

 // Copyright (c) 2011, 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.
 
 class Type implements Named, Hashable {
   final String name;
   bool isTested;
 
   /**
    * For core types (int, String, etc) this is the generated type assertion
@@ skipping 289 matching lines @@
   }
 }
 
 
 /** A type parameter within the body of the type. */
 class ParameterType extends Type {
   TypeParameter typeParameter;
   Type extendsType;
 
   bool get isClass() => false;