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

pkg/kernel/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 4226 matching lines @@
 /// bound.  A bound of `null` indicates that the bound has not been promoted and
 /// is the same as the [TypeParameter]'s bound.  This allows one to detect
 /// whether the bound has been promoted.
 class TypeParameterType extends DartType {
   TypeParameter parameter;
 
   /// An optional promoted bound on the type parameter.
   ///
   /// 'null' indicates that the type parameter's bound has not been promoted and
   /// is therefore the same as the bound of [parameter].
-  DartType bound;
+  DartType promotedBound;
 
-  TypeParameterType(this.parameter, [this.bound]);
+  TypeParameterType(this.parameter, [this.promotedBound]);
 
   accept(DartTypeVisitor v) => v.visitTypeParameterType(this);
 
   visitChildren(Visitor v) {}
 
   bool operator ==(Object other) {
     return other is TypeParameterType && parameter == other.parameter;
   }
 
   int get hashCode => _temporaryHashCodeTable[parameter] ?? parameter.hashCode;
+
+  /// Returns the bound of the type parameter, accounting for promotions.
+  DartType get bound => promotedBound ?? parameter.bound;
 }
 
 /// Declaration of a type variable.
 ///
 /// Type parameters declared in a [Class] or [FunctionNode] are part of the AST,
 /// have a parent pointer to its declaring class or function, and will be seen
 /// by tree visitors.
 ///
 /// Type parameters declared by a [FunctionType] are orphans and have a `null`
 /// parent pointer.  [TypeParameter] objects should not be shared between
@@ skipping 346 matching lines @@
 /// typedef has not been assigned a canonical name yet.
 ///
 /// Returns `null` if the typedef is `null`.
 CanonicalName getCanonicalNameOfTypedef(Typedef typedef_) {
   if (typedef_ == null) return null;
   if (typedef_.canonicalName == null) {
     throw '$typedef_ has no canonical name';
   }
   return typedef_.canonicalName;
 }