Cleanup dangling comment references.

Quick pass at fixing a few dangling references as revealed by the new `comment_references` lint (https://github.com/dart-lang/linter/issues/240).

There's a bunch more to do here before we can turn it on by default (~430 lints as of now).  Many of them are a simple matter of adding an import (e.g., `dart:async` for library docs that reference `Future`) but others will require a bit of thought.  Probably best done by the folks writing the code. :)

packages/flutter_markdown/lib/src/markdown_style.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'package:flutter/material.dart';
+import 'markdown.dart';
 import 'markdown_style_raw.dart';
 
 /// Style used for rendering markdown formatted text using the [MarkdownBody]

packages/flutter_markdown/lib/src/markdown_style_raw.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'package:flutter/widgets.dart';
+import 'markdown.dart';
 
 /// Style used for rendering markdown formatted text using the [MarkdownBody]
 /// widget.