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

pkg/compiler/lib/src/cps_ir/cps_ir_builder.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.
 
 library dart2js.ir_builder;
 
 import '../constants/constant_system.dart';
 import '../constants/expressions.dart';
 import '../constants/values.dart' show PrimitiveConstantValue;
 import '../dart_types.dart';
@@ skipping 2652 matching lines @@
                                      List<ir.Primitive> arguments) {
     return addPrimitive(new ir.CreateInvocationMirror(selector, arguments));
   }
 
   @override
   ir.Primitive buildTypeOperator(ir.Primitive value,
                                  DartType type,
                                  {bool isTypeTest}) {
     assert(isOpen);
     assert(isTypeTest != null);
+    // For type tests, we must treat specially the rare cases where `null`

[karlklose, 2015/05/28 07:42:30]

Move comment back into the if branch?

Also, the treatment of `null` is not the only special case that is handled here.
Maybe rewrite the comment to something like:
 // Handle statically known checks against [Object], [dynamic], which  
 // are trivial, and [Null], which can be decided by an identity    
 // comparison.

[asgerf, 2015/05/28 08:52:55]

It should be clear that this is not an optimization. It MUST be done because
`null` satisfies these tests and the codegen for TypeOperator assumes that null
can never satisfy a type test.

I'll try to clarify and move it back into the branch.

+    // satisfies the test (which otherwise never satisfies a type test).
     if (isTypeTest) {
-      // The TypeOperator node does not allow the Object, dynamic, and Null types,
-      // because the null value satisfies them. These must be handled here.
       if (type.isObject || type.isDynamic) {
         // `x is Object` and `x is dynamic` are always true, even if x is null.
         return buildBooleanConstant(true);
       }
       if (type is InterfaceType && type.element == program.nullClass) {
         // `x is Null` is true if and only if x is null.
         return addPrimitive(new ir.Identical(value, buildNullConstant()));
       }
     }
-    // Null cannot not satisfy the check. Use a standard subtype check.
     List<ir.Primitive> typeArguments = const <ir.Primitive>[];
     if (type is GenericType && type.typeArguments.isNotEmpty) {
       typeArguments = type.typeArguments.map(buildTypeExpression).toList();
+    } else if (type is TypeVariableType) {
+      typeArguments = <ir.Primitive>[buildTypeVariableAccess(type)];
     }
     ir.Primitive check = _continueWithExpression(
             (k) => new ir.TypeOperator(value,
                        type, typeArguments, k, isTypeTest: isTypeTest));
     return check;
   }
 }
 
 
 /// Location of a variable relative to a given closure.
@@ skipping 67 matching lines @@
 }
 
 /// Synthetic parameter to a JavaScript factory method that takes the type
 /// argument given for the type variable [variable].
 class TypeInformationParameter implements Local {
   final TypeVariableElement variable;
   final ExecutableElement executableContext;
   TypeInformationParameter(this.variable, this.executableContext);
   String get name => variable.name;
 }