Add a much simplified set implementation designed to waste little memory for small sets.

sdk/lib/_internal/compiler/implementation/inferrer/inferrer_visitor.dart

 // Copyright (c) 2013, 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.
 
 library inferrer_visitor;
 
 import '../dart2jslib.dart' hide Selector, TypedSelector;
 import '../dart_types.dart';
 import '../elements/elements.dart';
 import '../tree/tree.dart';
@@ skipping 121 matching lines @@
     variables[variable] = mask;
   }
 
   void forEachOwnLocal(void f(Element element, T type)) {
     if (variables == null) return;
     variables.forEach(f);
   }
 
   void forEachLocalUntilNode(Node node,
                              void f(Element element, T type),
-                             [Set<Element> seenLocals]) {
-    if (seenLocals == null) seenLocals = new Set<Element>();
+                             [Setlet<Element> seenLocals]) {
+    if (seenLocals == null) seenLocals = new Setlet<Element>();
     if (variables != null) {
       variables.forEach((element, type) {
         if (seenLocals.contains(element)) return;
         seenLocals.add(element);
         f(element, type);
       });
     }
     if (block == node) return;
     if (parent != null) parent.forEachLocalUntilNode(node, f, seenLocals);
   }
@@ skipping 985 matching lines @@
     return type;
   }
 
   T visitCascade(Cascade node) {
     // Ignore the result of the cascade send and return the type of the cascade
     // receiver.
     visit(node.expression);
     return cascadeReceiverStack.removeLast();
   }
 }