Make arm assembler explicitly take both registers for load/store-pair.

runtime/vm/assembler_arm.h

Index: runtime/vm/assembler_arm.h
diff --git a/runtime/vm/assembler_arm.h b/runtime/vm/assembler_arm.h
index 9ba279c5f571b57fc2a9cf5b28587524ea05d3fa..c40fd6b6899127b7b198224b5d6f8c0a6c7e1b8d 100644
--- a/runtime/vm/assembler_arm.h
+++ b/runtime/vm/assembler_arm.h
@@ -513,8 +513,11 @@ class Assembler : public ValueObject {
   // ldrd and strd actually support the full range of addressing modes, but
   // we don't use them, and we need to split them up into two instructions for
   // ARMv5TE, so we only support the base + offset mode.
-  void ldrd(Register rd, Register rn, int32_t offset, Condition cond = AL);
-  void strd(Register rd, Register rn, int32_t offset, Condition cond = AL);
+  // rd must be an even register and rd2 must be rd + 1.
+  void ldrd(Register rd, Register rd2, Register rn, int32_t offset,
+            Condition cond = AL);
+  void strd(Register rd, Register rd2, Register rn, int32_t offset,
+            Condition cond = AL);
 
   void ldm(BlockAddressMode am, Register base,
            RegList regs, Condition cond = AL);