[WIP] Classifies names according to case, uses for filename generation.

lib/src/dartdoc_options.dart

@@ -1131,7 +1131,7 @@ Future<List<DartdocOption>> createDartdocOptions() async {
     new DartdocOptionArgFile<List<String>>('includeExternal', null,
         isFile: true,
         help:
-            'Additional (external) dart files to include; use "dir/fileName", '
+            'Additional (external) dart files to include; use "dir/filename", '
             'as in lib/material.dart.',
         mustExist: true,
         splitCommas: true),

lib/src/html/html_generator.dart

@@ -79,9 +79,20 @@ class HtmlGenerator extends Generator {
       if (!enabled) {
         throw new StateError('`write` was called after `generate` completed.');
       }
-      // If you see this assert, we're probably being called to build non-canonical
-      // docs somehow.  Check data.self.isCanonical and callers for bugs.
-      assert(allowOverwrite || !writtenFiles.contains(filePath));
+
+      assert(
+          !writtenFiles
+              .map((String file) => file.toLowerCase())
+              .contains(filePath.toLowerCase()),
+          'Filename case metamer found. Output would not be correct on a '
+          'case-insensitive filesystem (like macOS). Metamer found was '
+          '"$filePath", which collides with '
+          '"${writtenFiles.where((String name) => name.toLowerCase() == filePath.toLowerCase()).join('" and "')}"');
+
+      assert(
+          allowOverwrite || !writtenFiles.contains(filePath),
+          'Being called to build non-canonical docs somehow.  Check '
+          'data.self.isCanonical and callers for bugs.');
 
       var file = new File(pathLib.join(outputDirectoryPath, filePath));
       var parent = file.parent;

lib/src/html/html_generator_instance.dart

@@ -228,7 +228,7 @@ class HtmlGeneratorInstance {
     }
     TemplateData data = new LibraryTemplateData(_options, packageGraph, lib);
 
-    _build(pathLib.join(lib.dirName, '${lib.fileName}'),
+    _build(pathLib.join(lib.dirName, '${lib.filename}'),
         _templates.libraryTemplate, data);
   }