code: @Override
      public void visitLineNumber(int line, Label start) {
          // If we haven't given a line number to the method entry, do so now.
          if (!methodEntryHasLineNumber) {
              methodEntryHasLineNumber = true;
              start = methodEntryLabel;
          }
  
          lineNumber = line;
          super.visitLineNumber(line, start);
      }
docs: Vist a line number block and its associated label. We will use this
  to keep track of (roughly) what line we're on.