Another try.

tools/testing/dart/test_suite.dart

 // Copyright (c) 2012, 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.
 
 /**
  * Classes and methods for enumerating and preparing tests.
  *
  * This library includes:
  *
  * - Creating tests by listing all the Dart files in certain directories,
@@ skipping 1499 matching lines @@
       base = new Path('/');
     }
     Directory dir = new Directory.fromPath(base);
     Expect.isTrue(dir.existsSync(),
                   "Expected ${dir} to already exist");
     var segments = relativePath.segments();
     for (String segment in segments) {
       // Ugly hack: if Windows absolute path starts with a drive,
       // rewrite [base] properly.
       if (segment.length == 2 && segment.endsWith(':')) {
-        base = new Path('${segment}\\');
+        base = new Path(segment);

[Bill Hesse, 2012/12/04 12:36:55]

I would use base = base.append(segment) here, like below.
Path('/').append('E:') is Path('/E:'), which is what you want as a path.  All
you want to do is skip the directory check and creation.

         continue;
       }
       base = base.append(segment);
       dir = new Directory.fromPath(base);
       print("======= creating $dir $base");

[Bill Hesse, 2012/12/04 12:36:55]

I would put the debug printing just before the dir.createSync(), so you know
when that is being hit.

       if (!dir.existsSync()) {
           dir.createSync();
       }
       Expect.isTrue(dir.existsSync(), "Failed to create ${dir.path}");
     }
     return dir;
   }
 
   /**
    * Copy a [source] file to a new place.
@@ skipping 145 matching lines @@
  * $pass tests are expected to pass
  * $failOk tests are expected to fail that we won't fix
  * $fail tests are expected to fail that we should fix
  * $crash tests are expected to crash that we should fix
  * $timeout tests are allowed to timeout
  * $compileErrorSkip tests are skipped on browsers due to compile-time error
 """;
     print(report);
    }
 }