Inline _OneByteString._setAt. The key knowledge is that _setAt is an internal method that has to be…

runtime/vm/intermediate_language_x64.cc

Index: runtime/vm/intermediate_language_x64.cc
===================================================================
--- runtime/vm/intermediate_language_x64.cc	(revision 22522)
+++ runtime/vm/intermediate_language_x64.cc	(working copy)
@@ -1328,6 +1328,7 @@
   ASSERT(idx == 2);
   switch (class_id_) {
     case kArrayCid:
+    case kOneByteStringCid:
     case kTypedDataInt8ArrayCid:
     case kTypedDataUint8ArrayCid:
     case kExternalTypedDataUint8ArrayCid:
@@ -1380,6 +1381,7 @@
     case kTypedDataInt8ArrayCid:
     case kTypedDataUint8ArrayCid:
     case kTypedDataUint8ClampedArrayCid:
+    case kOneByteStringCid:
       // TODO(fschneider): Add location constraint for byte registers (RAX,
       // RBX, RCX, RDX) instead of using a fixed register.
       locs->set_in(2, Location::FixedRegisterOrSmiConstant(value(), RAX));
@@ -1452,6 +1454,7 @@
     case kTypedDataInt8ArrayCid:
     case kTypedDataUint8ArrayCid:
     case kExternalTypedDataUint8ArrayCid:
+    case kOneByteStringCid:
       if (locs()->in(2).IsConstant()) {
         const Smi& constant = Smi::Cast(locs()->in(2).constant());
         __ movb(element_address,