Implement is-checks against type variables.

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

Index: sdk/lib/_internal/compiler/implementation/js_backend/runtime_types.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_backend/runtime_types.dart b/sdk/lib/_internal/compiler/implementation/js_backend/runtime_types.dart
index 746df0e8314b54715af2dd13d17c024902cec9ad..0e6acb1b658623cf2fb65cae9ec2cbd36bb01c24 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/runtime_types.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/runtime_types.dart
@@ -21,15 +21,6 @@ class RuntimeTypeInformation {
   /// instantiations and checks.
   Set<ClassElement> allArguments;
 
-  bool isJsNative(Element element) {
-    return (element == compiler.intClass ||
-            element == compiler.boolClass ||
-            element == compiler.numClass ||
-            element == compiler.doubleClass ||
-            element == compiler.stringClass ||
-            element == compiler.listClass);
-  }
-
   TypeChecks cachedRequiredChecks;
 
   TypeChecks getRequiredChecks() {
@@ -294,4 +285,14 @@ class TypeCheckMapping implements TypeChecks {
   }
 
   Iterator<ClassElement> get iterator => map.keys.iterator;
+
+  String toString() {
+    StringBuffer sb = new StringBuffer();
+    for (ClassElement holder in this) {
+      for (ClassElement check in [holder]) {
+        sb.add('${holder.name.slowToString()}.${check.name.slowToString()}, ');
+      }
+    }
+    return '[$sb]';
+  }
 }