Handle is checks (in a naive way: no narrowing for now).

sdk/lib/_internal/compiler/implementation/types/concrete_types_inferrer.dart

Index: sdk/lib/_internal/compiler/implementation/types/concrete_types_inferrer.dart
diff --git a/sdk/lib/_internal/compiler/implementation/types/concrete_types_inferrer.dart b/sdk/lib/_internal/compiler/implementation/types/concrete_types_inferrer.dart
index f1968bd02943e899a226f18eb38e278a220121a7..6c05203f9de40758087d2be4802042771106a075 100644
--- a/sdk/lib/_internal/compiler/implementation/types/concrete_types_inferrer.dart
+++ b/sdk/lib/_internal/compiler/implementation/types/concrete_types_inferrer.dart
@@ -1504,6 +1504,11 @@ class TypeInferrerVisitor extends ResolvedVisitor<ConcreteType> {
   }
 
   ConcreteType visitOperatorSend(Send node) {
+    SourceString name =
+        canonicalizeMethodName(node.selector.asIdentifier().source);
+    if (name == const SourceString('is')) {
+      return inferrer.singletonConcreteType(inferrer.baseTypes.boolBaseType);
+    }
     return visitDynamicSend(node);
   }