Implement is-checks with typedefs for callable objects.

sdk/lib/_internal/compiler/implementation/ssa/optimize.dart

Index: sdk/lib/_internal/compiler/implementation/ssa/optimize.dart
diff --git a/sdk/lib/_internal/compiler/implementation/ssa/optimize.dart b/sdk/lib/_internal/compiler/implementation/ssa/optimize.dart
index c83db8c522ac7ce6a6d433cad4e55b1e719e6a1c..b6ea705a87af1e3c376d4f94d0990109d6bc89fc 100644
--- a/sdk/lib/_internal/compiler/implementation/ssa/optimize.dart
+++ b/sdk/lib/_internal/compiler/implementation/ssa/optimize.dart
@@ -511,8 +511,10 @@ class SsaConstantFolder extends HBaseVisitor implements OptimizationPhase {
   HInstruction visitIs(HIs node) {
     DartType type = node.typeExpression;
     Element element = type.element;
-    if (identical(element.kind, ElementKind.TYPE_VARIABLE)) {
+    if (element.isTypeVariable()) {
       compiler.unimplemented("visitIs for type variables");
+    } if (element.isTypedef()) {
+      return node;
     }
 
     HType expressionType = types[node.expression];