public void addIntroductionIfNecessary(Insn insn) {
      RegisterSpec spec = locals.getAssignment(insn);

      if (spec != null) {
        addOutput(new LocalStart(insn.getPosition(), spec));
      }
    }
Adds a {@link LocalStart} to the output if the given instruction in fact introduces a local
variable.

@param insn {@code non-null;} instruction in question
android/android_framework/dex-maker/src/main/java/com/android/dx/dex/code/RopTranslator.java