public IntList getSubroutines() {
    return subroutines;
  }
Returns the largest subroutine nesting this block may be in. An empty list is returned if this
block is not in any subroutine. Subroutines are identified by the label of their start block.
The list is ordered such that the deepest nesting (the actual subroutine this block is in) is
the last label in the list.

@return {@code non-null;} list as noted above
android/android_framework/dex-maker/src/main/java/com/android/dx/cf/code/Frame.java