Avoid array bounds check when allowed by guarded field.

runtime/vm/flow_graph_optimizer.cc

Index: runtime/vm/flow_graph_optimizer.cc
diff --git a/runtime/vm/flow_graph_optimizer.cc b/runtime/vm/flow_graph_optimizer.cc
index ba9a4d9fa9dbe68442e61735fcc7bd37da568e57..b2fc5b6ee596759c38c9a29df30d17651b045fd2 100644
--- a/runtime/vm/flow_graph_optimizer.cc
+++ b/runtime/vm/flow_graph_optimizer.cc
@@ -770,6 +770,13 @@ intptr_t FlowGraphOptimizer::PrepareIndexedOp(InstanceCallInstr* call,
                call->env(),
                Definition::kEffect);
 
+  // Get the field if the array is loaded from a class field.
+  const Field* field = NULL;
+  if ((*array)->IsLoadField()) {
+    LoadFieldInstr* load_field_instr = (*array)->AsLoadField();
+    field = load_field_instr->field();
+  }
+
   // Insert array length load and bounds check.
   const bool is_immutable =
       CheckArrayBoundInstr::IsFixedLengthArrayType(class_id);
@@ -783,7 +790,8 @@ intptr_t FlowGraphOptimizer::PrepareIndexedOp(InstanceCallInstr* call,
       LoadFieldInstr::RecognizedKindFromArrayCid(class_id));
   InsertBefore(call, length, NULL, Definition::kValue);
   InsertBefore(call,
-               new CheckArrayBoundInstr(new Value(length),
+               new CheckArrayBoundInstr(field,
+                                        new Value(length),
                                         new Value(*index),
                                         call->deopt_id()),
                call->env(),
@@ -1688,7 +1696,8 @@ LoadIndexedInstr* FlowGraphOptimizer::BuildStringCodeUnitAt(
     LoadFieldInstr* length = BuildLoadStringLength(str);
     InsertBefore(call, length, NULL, Definition::kValue);
     InsertBefore(call,
-                 new CheckArrayBoundInstr(new Value(length),
+                 new CheckArrayBoundInstr(NULL,
+                                          new Value(length),
                                           new Value(index),
                                           call->deopt_id()),
                  call->env(),
@@ -2461,7 +2470,8 @@ void FlowGraphOptimizer::PrepareByteArrayViewOp(
 
     // Check byte_index < len_in_bytes.
   InsertBefore(call,
-               new CheckArrayBoundInstr(new Value(len_in_bytes),
+               new CheckArrayBoundInstr(NULL,
+                                        new Value(len_in_bytes),
                                         new Value(byte_index),
                                         call->deopt_id()),
                call->env(),