dart2js cps: Track underlying value of interceptors in type propagation.

pkg/compiler/lib/src/cps_ir/type_propagation.dart

Index: pkg/compiler/lib/src/cps_ir/type_propagation.dart
diff --git a/pkg/compiler/lib/src/cps_ir/type_propagation.dart b/pkg/compiler/lib/src/cps_ir/type_propagation.dart
index 8b7141f1303cf5c31c266239a4a9af1509b6fc2f..87f9abeb58e426b238675eff6af539112b913ace 100644
--- a/pkg/compiler/lib/src/cps_ir/type_propagation.dart
+++ b/pkg/compiler/lib/src/cps_ir/type_propagation.dart
@@ -839,6 +839,10 @@ class _TypePropagationVisitor<T> implements Visitor {
 
   void visitInterceptor(Interceptor node) {
     setReachable(node.input.definition);
+    _AbstractValue<T> value = getValue(node.input.definition);
+    if (!value.isNothing) {
+      setValue(node, nonConstant());
+    }
   }
 
   void visitGetField(GetField node) {