Implement is-checks against type variables.

sdk/lib/_internal/compiler/implementation/js_backend/backend.dart

 // Copyright (c) 2012, 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.
 
 part of js_backend;
 
 typedef void Recompile(Element element);
 
 class ReturnInfo {
   HType returnType;
@@ skipping 972 matching lines @@
     enqueueInResolution(getTraceFromException());
   }
 
   void registerRuntimeType() {
     enqueueInResolution(getSetRuntimeTypeInfo());
     enqueueInResolution(getGetRuntimeTypeInfo());
     enqueueInResolution(getGetRuntimeTypeArgument());
   }
 
   void registerIsCheck(DartType type, Enqueuer world) {
-    if (!type.isRaw) {
+    if (!type.isRaw || type is TypeVariableType) {

[ngeoffray, 2013/02/19 09:00:41]

Banned is check :)

Do you really need all the following just because we have (a is T) ? What about
cases where we never allocate generic things.

       enqueueInResolution(getSetRuntimeTypeInfo());
       enqueueInResolution(getGetRuntimeTypeInfo());
       enqueueInResolution(getGetRuntimeTypeArgument());
       enqueueInResolution(getCheckArguments());
+      enqueueInResolution(getGetObjectIsSubtype());
     }
     // [registerIsCheck] is also called for checked mode checks, so we
     // need to register checked mode helpers.
     if (compiler.enableTypeAssertions) {
       SourceString helperName = getCheckedModeHelper(type);
       Element e = compiler.findHelper(helperName);
       if (e != null) world.addToWorkList(e);
       // We also need the native variant of the check (for DOM types).
       helperName = nativeNames[helperName.stringValue];
       if (helperName != null) {
@@ skipping 412 matching lines @@
   }
 
   Element getGetRuntimeTypeArgument() {
     return compiler.findHelper(const SourceString('getRuntimeTypeArgument'));
   }
 
   Element getCheckArguments() {
     return compiler.findHelper(const SourceString('checkArguments'));
   }
 
+  Element getGetObjectIsSubtype() {
+    return compiler.findHelper(const SourceString('objectIsSubtype'));
+  }
+
   Element getThrowNoSuchMethod() {
     return compiler.findHelper(const SourceString('throwNoSuchMethod'));
   }
 
   Element getCreateRuntimeType() {
     return compiler.findHelper(const SourceString('createRuntimeType'));
   }
 
   Element getFallThroughError() {
     return compiler.findHelper(const SourceString("getFallThroughError"));
@@ skipping 13 matching lines @@
    *
    * Invariant: [element] must be a declaration element.
    */
   void eagerRecompile(Element element) {
     assert(invariant(element, element.isDeclaration));
     generatedCode.remove(element);
     generatedBailoutCode.remove(element);
     compiler.enqueuer.codegen.addToWorkList(element);
   }
 }