dart2js cps: 'is'-check directly against type variables.

pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart

 // Copyright (c) 2013, 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.
 
 // IrNodes are kept in a separate library to have precise control over their
 // dependencies on other parts of the system.
 library dart2js.ir_nodes;
 
 import '../constants/expressions.dart';
 import '../constants/values.dart' as values show ConstantValue;
@@ skipping 386 matching lines @@
   accept(Visitor visitor) => visitor.visitInvokeConstructor(this);
 }
 
 /// "as" casts and "is" checks.
 // We might want to turn "is"-checks into a [Primitive] as it can never diverge.
 // But then we need to special-case for is-checks with an erroneous .type as
 // these will throw.
 class TypeOperator extends Expression {
   Reference<Primitive> value;
   final DartType type;
-  /// Type arguments to [type]. Since [type] may reference type variables in the
-  /// enclosing class, these are not constant.
+
+  /// If [type] is a [GenericType], this holds the internal representation of
+  /// the type arguments to [type]. Since these may reference type variables
+  /// from the enclosing class, they are not constant.
+  ///
+  /// If [type] is a [TypeVariableType], this is a singleton list with
+  /// the internal representation of the type held in that type variable.
+  ///
+  /// Otherwise the list is empty.
   final List<Reference<Primitive>> typeArguments;
   final Reference<Continuation> continuation;
   // TODO(johnniwinther): Use `Operator` class to encapsule the operator type.
   final bool isTypeTest;
 
   TypeOperator(Primitive value,
                this.type,
                List<Primitive> typeArguments,
                Continuation cont,
                {bool this.isTypeTest})
@@ skipping 995 matching lines @@
     processNonTailThrow(node);
     processReference(node.value);
   }
 
   processCreateInvocationMirror(CreateInvocationMirror node) {}
   visitCreateInvocationMirror(CreateInvocationMirror node) {
     processCreateInvocationMirror(node);
     node.arguments.forEach(processReference);
   }
 }