Fix bug in flow-graph loop detection.

runtime/vm/flow_graph.h

Index: runtime/vm/flow_graph.h
===================================================================
--- runtime/vm/flow_graph.h	(revision 31954)
+++ runtime/vm/flow_graph.h	(working copy)
@@ -247,7 +247,9 @@
   // Find the natural loop for the back edge m->n and attach loop
   // information to block n (loop header). The algorithm is described in
   // "Advanced Compiler Design & Implementation" (Muchnick) p192.
-  void FindLoop(BlockEntryInstr* m, BlockEntryInstr* n);
+  // Returns a BitVector indexed by block pre-order number where each bit
+  // indicates membership in the loop.
+  BitVector* FindLoop(BlockEntryInstr* m, BlockEntryInstr* n);
 
   // Finds natural loops in the flow graph and attaches a list of loop
   // body blocks for each loop header.