Adds load/store from signed unscaled 9-bit offset w/ no writeback.

runtime/vm/disassembler_arm64.cc

Index: runtime/vm/disassembler_arm64.cc
===================================================================
--- runtime/vm/disassembler_arm64.cc	(revision 35221)
+++ runtime/vm/disassembler_arm64.cc	(working copy)
@@ -195,28 +195,28 @@
     Print("]");
   } else {
     switch (instr->Bits(10, 2)) {
-      case 1: {
+      case 0: {
+        // rn + signed 9-bit immediate, pre-index, no writeback.
         const int32_t imm9 = instr->SImm9Field();
-        // rn + signed 9-bit immediate, post-index, writeback.
         Print("[");
         PrintRegister(rn, R31IsSP);
+        buffer_pos_ += OS::SNPrint(current_position_in_buffer(),
+                           remaining_size_in_buffer(),
+                           ", #%d",
+                           imm9);
         Print("]");
-        buffer_pos_ += OS::SNPrint(current_position_in_buffer(),
-                                   remaining_size_in_buffer(),
-                                   ", #%d !",
-                                   imm9);
         break;
       }
-      case 3: {
+      case 1: {
         const int32_t imm9 = instr->SImm9Field();
-        // rn + signed 9-bit immediate, pre-index, writeback.
+        // rn + signed 9-bit immediate, post-index, writeback.
         Print("[");
         PrintRegister(rn, R31IsSP);
+        Print("]");
         buffer_pos_ += OS::SNPrint(current_position_in_buffer(),
                                    remaining_size_in_buffer(),
-                                   ", #%d",
+                                   ", #%d !",
                                    imm9);
-        Print("] !");
         break;
       }
       case 2: {
@@ -237,6 +237,18 @@
         Print("]");
         break;
       }
+      case 3: {
+        const int32_t imm9 = instr->SImm9Field();
+        // rn + signed 9-bit immediate, pre-index, writeback.
+        Print("[");
+        PrintRegister(rn, R31IsSP);
+        buffer_pos_ += OS::SNPrint(current_position_in_buffer(),
+                                   remaining_size_in_buffer(),
+                                   ", #%d",
+                                   imm9);
+        Print("] !");
+        break;
+      }
       default: {
         Print("???");
       }