code: public @CheckForNull
      SourceLineRange getFieldLine(String className, String fieldName) {
          return fieldLineMap.get(new FieldDescriptor(className, fieldName));
      }
docs: Look up the line number range for a field.
  
  @param className
  name of class containing the field
  @param fieldName
  name of field
  @return the line number range, or null if no line number is known for the
  field