Use predicates to check simple function types.

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

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of js_backend;
 
 /**
  * Assigns JavaScript identifiers to Dart variables, class-names and members.
  */
 class Namer implements ClosureNamer {
@@ skipping 1222 matching lines @@
 
   visit(DartType type) {
     type.accept(this, null);
   }
 
   visitType(DartType type, _) {
     sb.write(type.name.slowToString());
   }
 
   visitFunctionType(FunctionType type, _) {
+    if (type.isSimple) {
+      sb.write('args${type.parameterTypes.slowLength()}');
+      return;
+    }
     visit(type.returnType);
     sb.write('_');
     for (Link<DartType> link = type.parameterTypes;
          !link.isEmpty;
          link = link.tail) {
       sb.write('_');
       visit(link.head);
     }
     bool first = false;
     for (Link<DartType> link = type.optionalParameterTypes;
@@ skipping 14 matching lines @@
         if (!first) {
           sb.write('_');
         }
         sb.write('_');
         visit(link.head);
         first = true;
       }
     }
   }
 }