Support use of external strings as inputs to LoadCodeUnitsInstr.

runtime/vm/assembler_mips.h

Index: runtime/vm/assembler_mips.h
diff --git a/runtime/vm/assembler_mips.h b/runtime/vm/assembler_mips.h
index 1a73e4443157fc0306542179cb7114f212ebd96f..6e12f0b90447376e3547d0f2f5f23f52246cbc20 100644
--- a/runtime/vm/assembler_mips.h
+++ b/runtime/vm/assembler_mips.h
@@ -1163,6 +1163,10 @@ class Assembler : public ValueObject {
     }
   }
 
+  void LoadFieldFromOffset(Register reg, Register base, int32_t offset) {
+    LoadFromOffset(reg, base, offset - kHeapObjectTag);
+  }
+
   void StoreToOffset(Register reg, Register base, int32_t offset) {
     ASSERT(!in_delay_slot_);
     if (Utils::IsInt(kImmBits, offset)) {