Fix coverage tool on Windows

tools/coverage.dart

 // Copyright (c) 2013, 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.
 
 // This test forks a second vm process that runs a dart script as
 // a debug target, single stepping through the entire program, and
 // recording each breakpoint. At the end, a coverage map of the source
 // is printed.
 //
 // Usage: dart coverage.dart [--wire] [--verbose] target_script.dart
@@ skipping 83 matching lines @@
     print(url);
     tokenCounts.forEach((tp, bpCount) {
       int lineNumber = tokenPosToLine[tp];
       var lineCount = lineCounts[lineNumber];
       // Remember maximum breakpoint count of all tokens in this line.
       if (lineCount == null || lineCount < bpCount) {
         lineCounts[lineNumber] = bpCount;
       }
     });
 
-    List lines = new File(Uri.parse(url).path).readAsLinesSync();
+    String srcPath = new Path(Uri.parse(url).path).toNativePath();
+    List lines = new File(srcPath).readAsLinesSync();
     for (int line = 1; line <= lines.length; line++) {
       String prefix = "      ";
       if (lineCounts.containsKey(line)) {
         prefix = lineCounts[line].toString();
         StringBuffer b = new StringBuffer();
         for (int i = prefix.length; i < 6; i++) b.write(" ");
         b.write(prefix);
         prefix = b.toString();
       }
       print("${prefix}|${lines[line-1]}");
@@ skipping 369 matching lines @@
         targetOpts.add(str);
         break;
     }
   }
 
   Process.start(options.executable, targetOpts).then((Process process) {
     process.stdin.close();
     var debugger = new Debugger(process);
   });
 }