Revert parts of r45502 and always generate guard field instructions.

runtime/vm/intermediate_language_x64.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/globals.h"  // Needed here to get TARGET_ARCH_X64.
 #if defined(TARGET_ARCH_X64)
 
 #include "vm/intermediate_language.h"
 
 #include "vm/dart_entry.h"
@@ skipping 1426 matching lines @@
 
   return summary;
 }
 
 
 void GuardFieldClassInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
   const intptr_t value_cid = value()->Type()->ToCid();
   const intptr_t field_cid = field().guarded_cid();
   const intptr_t nullability = field().is_nullable() ? kNullCid : kIllegalCid;
 
-  ASSERT(field_cid != kDynamicCid);
+  if (field_cid == kDynamicCid) {
+    ASSERT(!compiler->is_optimizing());
+    return;  // Nothing to emit.
+  }
 
   const bool emit_full_guard =
       !compiler->is_optimizing() || (field_cid == kIllegalCid);
 
   const bool needs_value_cid_temp_reg =
       (value_cid == kDynamicCid) && (emit_full_guard || (field_cid != kSmiCid));
 
   const bool needs_field_temp_reg = emit_full_guard;
 
   const Register value_reg = locs()->in(0).reg();
@@ skipping 117 matching lines @@
     LocationSummary* summary = new(zone) LocationSummary(
         zone, kNumInputs, 0, LocationSummary::kNoCall);
     summary->set_in(0, Location::RequiresRegister());
     return summary;
   }
   UNREACHABLE();
 }
 
 
 void GuardFieldLengthInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
-  ASSERT(field().guarded_list_length() != Field::kNoFixedLength);
+  if (field().guarded_list_length() == Field::kNoFixedLength) {
+    ASSERT(!compiler->is_optimizing());
+    return;  // Nothing to emit.
+  }
 
   Label* deopt = compiler->is_optimizing() ?
       compiler->AddDeoptStub(deopt_id(), ICData::kDeoptGuardField) : NULL;
 
   const Register value_reg = locs()->in(0).reg();
 
   if (!compiler->is_optimizing() ||
       (field().guarded_list_length() == Field::kUnknownFixedLength)) {
     const Register field_reg = locs()->temp(0).reg();
     const Register offset_reg = locs()->temp(1).reg();
@@ skipping 4847 matching lines @@
   __ Drop(1);
   __ popq(result);
 }
 
 
 }  // namespace dart
 
 #undef __
 
 #endif  // defined TARGET_ARCH_X64