Compute the def-use list on-demand by walking the dominator tree.

runtime/vm/flow_graph_allocator.cc

Index: runtime/vm/flow_graph_allocator.cc
diff --git a/runtime/vm/flow_graph_allocator.cc b/runtime/vm/flow_graph_allocator.cc
index dc10bb7888124bd449bec47d9670d97599f9d0e9..a9285f73a5262f687b20434e25f854766ccd519e 100644
--- a/runtime/vm/flow_graph_allocator.cc
+++ b/runtime/vm/flow_graph_allocator.cc
@@ -102,10 +102,7 @@ void FlowGraphAllocator::EliminateEnvironmentUses() {
 
           PushArgumentInstr* push_argument = def->AsPushArgument();
           if ((push_argument != NULL) && push_argument->WasEliminated()) {
-            // TODO(zerny): This should be unreachable if we could properly
-            // replace uses by values. (See RemovePushArguments in
-            // flow_graph_optimizer.cc).
-            (*values)[i] = push_argument->value();
+            (*values)[i] = push_argument->value()->CopyValue();
             continue;
           }