Clean up handling of guarded fields in the flow graph builder.

runtime/vm/flow_graph.cc

Index: runtime/vm/flow_graph.cc
===================================================================
--- runtime/vm/flow_graph.cc	(revision 27696)
+++ runtime/vm/flow_graph.cc	(working copy)
@@ -38,11 +38,28 @@
     licm_allowed_(true),
     use_far_branches_(false),
     loop_headers_(NULL),
-    loop_invariant_loads_(NULL) {
+    loop_invariant_loads_(NULL),
+    guarded_fields_(builder.guarded_fields()) {
   DiscoverBlocks();
 }
 
 
+void FlowGraph::AddToGuardedFields(
+    const Field* field,
+    ZoneGrowableArray<const Field*>* array) {
+  if ((field->guarded_cid() == kDynamicCid) ||
+      (field->guarded_cid() == kIllegalCid)) {
+    return;
+  }
+  for (intptr_t j = 0; j < array->length(); j++) {
+    if ((*array)[j]->raw() == field->raw()) {
+      return;
+    }
+  }
+  array->Add(field);
+}
+
+
 GrowableArray<BlockEntryInstr*>* FlowGraph::codegen_block_order(
     bool is_optimized) {
   return (is_optimized && FLAG_reorder_basic_blocks)