dangling_library_doc_comments   
                  Dangling library doc comment.
Description
#
                    The analyzer produces this diagnostic when a documentation comment that
                    appears to be library documentation isn't followed by a library
                    directive. More specifically, it is produced when a documentation comment
                    appears before the first directive in the library, assuming that it isn't
                    a library directive, or before the first top-level declaration and is
                    separated from the declaration by one or more blank lines.
                  
Example
#
                    The following code produces this diagnostic because there's a
                    documentation comment before the first import directive:
                  
/// This is a great library.
import 'dart:core';
The following code produces this diagnostic because there's a documentation comment before the first class declaration, but there's a blank line between the comment and the declaration.
/// This is a great library.
class C {}
Common fixes
#
                    If the comment is library documentation, then add a library directive
                    without a name:
                  
/// This is a great library.
library;
import 'dart:core';
If the comment is documentation for the following declaration, then remove the blank line:
/// This is a great library.
class C {}