Make the kernel API for promoted type parameter bounds less error-prone.

pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart

 // Copyright (c) 2017, 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 file declares a "shadow hierarchy" of concrete classes which extend
 /// the kernel class hierarchy, adding methods and fields needed by the
 /// BodyBuilder.
 ///
 /// Instances of these classes may be created using the factory methods in
 /// `ast_factory.dart`.
@@ skipping 1919 matching lines @@
       // implemented, so just skip it for now.
       // TODO(paulberry): once the BodyBuilder uses shadow classes for
       // everything, this case should no longer be needed.
     }
   }
 
   /// If the given [type] is a [TypeParameterType], resolve it to its bound.
   DartType resolveTypeParameter(DartType type) {
     DartType resolveOneStep(DartType type) {
       if (type is TypeParameterType) {
-        return type.bound ?? type.parameter.bound;
+        return type.bound;
       } else {
         return null;
       }
     }
 
     var resolved = resolveOneStep(type);
     if (resolved == null) return type;
 
     // Detect circularities using the tortoise-and-hare algorithm.
     type = resolved;
@@ skipping 265 matching lines @@
   }
 
   transformChildren(v) {
     return internalError("Internal error: Unsupported operation.");
   }
 
   visitChildren(v) {
     return internalError("Internal error: Unsupported operation.");
   }
 }