Fix build breakage caused by the recent toString() commit.

tests/corelib/src/CollectionToStringTest.dart

Index: tests/corelib/src/CollectionToStringTest.dart
diff --git a/tests/corelib/src/CollectionToStringTest.dart b/tests/corelib/src/CollectionToStringTest.dart
index 75161bb3e034b9b8bf4037cb074bc7e546f737f3..fae9c50779d262b8105692cea08d5f6ec2537985 100644
--- a/tests/corelib/src/CollectionToStringTest.dart
+++ b/tests/corelib/src/CollectionToStringTest.dart
@@ -254,7 +254,7 @@ Map populateRandomMap(int size, bool exact, StringBuffer stringRep,
  * If exact is true, the returned element will not be, and will not contain
  * a collection with ill-defined iteration order (i.e., a HashSet or HashMap).
  */
-void randomElement(int size, bool exact, StringBuffer stringRep,
+Object randomElement(int size, bool exact, StringBuffer stringRep,

[jjb, 2012/02/22 04:55:06]

I find it astonishing that the compiler never told me about this!  I did a lot
testing, with type-checking enabled.

     List beingMade) {
   Object result;
   double elementTypeFrac = Math.random();