Don't fix up paths that are already absolute.

utils/dartdoc/files.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // Functions for working with files and paths.
 
 /** The path to the file currently being written to, relative to [outdir]. */
 String _filePath;
 
 /** The file currently being written to. */
@@ skipping 19 matching lines @@
 endFile() {
   String outPath = '$_outdir/$_filePath';
   world.files.createDirectory(dirname(outPath), recursive: true);
 
   world.files.writeString(outPath, _file.toString());
   _filePath = null;
   _file = null;
 }
 
 /**
- * Converts [absolute] which is understood to be a full path from the root of
+ * Converts [fullPath] which is understood to be a full path from the root of
  * the generated docs to one relative to the current file.
  */
-String relativePath(String absolute) {
+String relativePath(String fullPath) {
+  // Don't make it relative if it's an absolute path.
+  if (isAbsolute(fullPath)) return fullPath;
+
   // TODO(rnystrom): Walks all the way up to root each time. Shouldn't do this
   // if the paths overlap.
-  return repeat('../', countOccurrences(_filePath, '/')) + absolute;
+  return repeat('../', countOccurrences(_filePath, '/')) + fullPath;
+}
+
+/** Gets whether or not the given URL is absolute or relative. */
+bool isAbsolute(String url) {
+  // TODO(rnystrom): This is a bit hackish. We consider any URL that lacks
+  // a scheme to be relative.
+  return const RegExp(@'^\w+:').hasMatch(url);
 }
 
 /** Gets the URL to the documentation for [library]. */
 libraryUrl(Library library) => '${sanitize(library.name)}.html';
 
 /** Gets the URL for the documentation for [type]. */
 typeUrl(Type type) {
   // Always get the generic type to strip off any type parameters or arguments.
   // If the type isn't generic, genericType returns `this`, so it works for
   // non-generic types too.
   return '${sanitize(type.library.name)}/${type.genericType.name}.html';
 }
 
 /** Gets the URL for the documentation for [member]. */
 memberUrl(Member member) => '${typeUrl(member.declaringType)}#${member.name}';
 
 /** Gets the anchor id for the document for [member]. */
 memberAnchor(Member member) => '${member.name}';