Add an optional bound to type parameter references

pkg/kernel/lib/ast.dart

Index: pkg/kernel/lib/ast.dart
diff --git a/pkg/kernel/lib/ast.dart b/pkg/kernel/lib/ast.dart
index fa2d18086786c490994e0d73ab078909a8e60dbc..72ab24dc7fb1254fbc1a7a137b64738752b29b35 100644
--- a/pkg/kernel/lib/ast.dart
+++ b/pkg/kernel/lib/ast.dart
@@ -4103,10 +4103,21 @@ class NamedType extends Node implements Comparable<NamedType> {
 final Map<TypeParameter, int> _temporaryHashCodeTable = <TypeParameter, int>{};
 
 /// Reference to a type variable.
+///
+/// A type variable has an optional bound because type promotion can change the
+/// 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;
 
-  TypeParameterType(this.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;
+
+  TypeParameterType(this.parameter, [this.bound]);
 
   accept(DartTypeVisitor v) => v.visitTypeParameterType(this);