Use WeakMap to support Expando if available.

tests/corelib/expando_test.dart

 // Copyright (c) 2012, 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.
 
 import "package:expect/expect.dart";
 
 class ExpandoTest {
   static Expando<int> visits;
 
   static testMain() {
     visits = new Expando<int>('visits');
     var legal = [ new Object(),
                   new List(), [1,2,3], const [1,2,3],
                   new Map(), {'x':1,'y':2}, const {'x':1,'y':2},
                   new Expando(), new Expando('horse') ];
     for (var object in legal) {
       testNamedExpando(object);
       testUnnamedExpando(object);
     }
     for (var object in legal) {
-      Expect.equals(2, visits[object]);
+      Expect.equals(2, visits[object], "$object");
     }
     testIllegal();
     testIdentity();
   }
 
   static visit(object) {
     int count = visits[object];
     count = (count == null) ? 1 : count + 1;
     visits[object] = count;
   }
@@ skipping 26 matching lines @@
     alternative[object] = 87;
     Expect.isNull(expando[object]);
     expando[object] = 99;
     Expect.equals(99, expando[object]);
     Expect.equals(87, alternative[object]);
   }
 
   static testIllegal() {
     Expando<int> expando = new Expando<int>();
     Expect.throws(() => expando[null], (exception)
-                  => exception is ArgumentError);
+                  => exception is ArgumentError, "null");
     Expect.throws(() => expando['string'], (exception)
-                  => exception is ArgumentError);
+                  => exception is ArgumentError, "'string'");
     Expect.throws(() => expando['string'], (exception)
-                  => exception is ArgumentError);
+                  => exception is ArgumentError, "'string'");
     Expect.throws(() => expando[42], (exception)
-                  => exception is ArgumentError);
+                  => exception is ArgumentError, "42");
     Expect.throws(() => expando[42.87], (exception)
-                  => exception is ArgumentError);
+                  => exception is ArgumentError, "42.87");
     Expect.throws(() => expando[true], (exception)
-                  => exception is ArgumentError);
+                  => exception is ArgumentError, "true");
     Expect.throws(() => expando[false], (exception)
-                  => exception is ArgumentError);
+                  => exception is ArgumentError, "false");
   }
 
   static testIdentity() {
     // Expando only depends on identity of object.
     Expando<int> expando = new Expando<int>();
     var m1 = new Mutable(1);
     var m2 = new Mutable(7);
     var m3 = new Mutable(13);
     expando[m1] = 42;
     Expect.equals(42, expando[m1]);
     m1.id = 37;
     Expect.equals(42, expando[m1]);
     expando[m2] = 37;
     expando[m3] = 10;
     m3.id = 1;
     Expect.equals(42, expando[m1]);
     Expect.equals(37, expando[m2]);
     Expect.equals(10, expando[m3]);
   }
 }
 
 main() => ExpandoTest.testMain();
 
 class Mutable {
   int id;
   Mutable(this.id);
   int get hashCode => id;
   bool operator==(other) => other is Mutable && other.id == id;
 }