Don't use a map literal if we insert non-string keys, otherwise it fails in checked mode.

tests/language/bailout5_test.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.
 
 // Test to make sure the bailout environment in dart2js is correct.
 
 var global;
 
 class A {
   var array;
 
   initArray() {
-    return global[0] == null ? [null] : {}; 
+    return global[0] == null ? [null] : new Map(); 
   }
 
   bar() {
     array = initArray();
     do {
       var element = array[0]; // bailout here
       if (element is Map) continue;
       if (element == null) break;
     } while (true);
     return global[0]; // bailout here
@@ skipping 10 matching lines @@
   }
 }
 
 
 void main() {
   global = [1];
   for (int i = 0; i < 2; i++) {
     Expect.equals(1, new A().baz());
     Expect.equals(1, new A().bar());
   }
-  global = {};
+  global = new Map();
   for (int i = 0; i < 2; i++) {
     Expect.equals(null, new A().baz());
     Expect.equals(null, new A().bar());
   }
 
   global[0] = 42;
   for (int i = 0; i < 2; i++) {
     Expect.equals(42, new A().baz());
     Expect.equals(42, new A().bar());
   }
 }