Avoid passing an invalid file URI to analyzer on Windows.

lib/src/dart_formatter.dart

 // Copyright (c) 2014, 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.
 
 library dart_style.src.dart_formatter;
 
 import 'dart:math' as math;
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 34 matching lines @@
   DartFormatter({this.lineEnding, int pageWidth, this.indent: 0})
       : this.pageWidth = (pageWidth == null) ? 80 : pageWidth;
 
   /// Formats the given [source] string containing an entire Dart compilation
   /// unit.
   ///
   /// If [uri] is given, it is a [String] or [Uri] used to identify the file
   /// being formatted in error messages.
   String format(String source, {uri}) {
     if (uri == null) {
-      uri = "<unknown>";
+      // Do nothing.
     } else if (uri is Uri) {
       uri = uri.toString();
     } else if (uri is String) {
       // Do nothing.
     } else {
       throw new ArgumentError("uri must be `null`, a Uri, or a String.");
     }
 
     return formatSource(
             new SourceCode(source, uri: uri, isCompilationUnit: true))
@@ skipping 59 matching lines @@
       }
     }
 
     errorListener.throwIfErrors();
 
     // Format it.
     var visitor = new SourceVisitor(this, lineInfo, source);
     return visitor.run(node);
   }
 }