Look up the correct constructor for default classes.

lib/compiler/implementation/elements/elements.dart

Index: lib/compiler/implementation/elements/elements.dart
diff --git a/lib/compiler/implementation/elements/elements.dart b/lib/compiler/implementation/elements/elements.dart
index c1da9dc56336a3824608c1280028f9d14e37012b..a9c34cbc8b6d9097bdb7982aad62141ac5e9f22d 100644
--- a/lib/compiler/implementation/elements/elements.dart
+++ b/lib/compiler/implementation/elements/elements.dart
@@ -825,6 +825,16 @@ class ClassElement extends ContainerElement {
     forEachMember(fieldFilter, includeBackendMembers, includeSuperMembers);
   }
 
+  bool implementsInterface(ClassElement intrface) {
+    for (Type implementedInterfaceType in allSupertypes) {
+      ClassElement implementedInterface = implementedInterfaceType.element;
+      if (implementedInterface === intrface) {
+        return true;
+      }
+    }
+    return false;
+  }
+
   bool isInterface() => false;
   bool isNative() => nativeName != null;
   SourceString nativeName;