Implement is-checks against type variables.

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

Index: sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
index 3bc06c91da6a14d4f45bf466365ad3a304afe449..ceb5eb4a5f58c0db4d95c556b268c13f75b36da2 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
@@ -988,6 +988,14 @@ class JavaScriptBackend extends Backend {
     enqueueInResolution(getTraceFromException());
   }
 
+  void registerSetRuntimeType() {
+    enqueueInResolution(getSetRuntimeTypeInfo());
+  }
+
+  void registerGetRuntimeTypeArgument() {
+    enqueueInResolution(getGetRuntimeTypeArgument());
+  }
+
   void registerRuntimeType() {
     enqueueInResolution(getSetRuntimeTypeInfo());
     enqueueInResolution(getGetRuntimeTypeInfo());
@@ -995,11 +1003,13 @@ class JavaScriptBackend extends Backend {
   }
 
   void registerIsCheck(DartType type, Enqueuer world) {
-    if (!type.isRaw) {
+    bool isTypeVariable = type.kind == TypeKind.TYPE_VARIABLE;
+    if (!type.isRaw || isTypeVariable) {
       enqueueInResolution(getSetRuntimeTypeInfo());
       enqueueInResolution(getGetRuntimeTypeInfo());
       enqueueInResolution(getGetRuntimeTypeArgument());
       enqueueInResolution(getCheckArguments());
+      if (isTypeVariable) enqueueInResolution(getGetObjectIsSubtype());
     }
     // [registerIsCheck] is also called for checked mode checks, so we
     // need to register checked mode helpers.
@@ -1444,6 +1454,10 @@ class JavaScriptBackend extends Backend {
     return compiler.findHelper(const SourceString('checkArguments'));
   }
 
+  Element getGetObjectIsSubtype() {
+    return compiler.findHelper(const SourceString('objectIsSubtype'));
+  }
+
   Element getThrowNoSuchMethod() {
     return compiler.findHelper(const SourceString('throwNoSuchMethod'));
   }