Don't use Uri's isAbsolute to detect if the path segment is absolute, in toFilePath.

sdk/lib/core/uri.dart

Index: sdk/lib/core/uri.dart
diff --git a/sdk/lib/core/uri.dart b/sdk/lib/core/uri.dart
index 503509946adfbaf2773a5809d73c837396c68882..f80a30018e9e14680c63fd30b9aebbe8f952d9b3 100644
--- a/sdk/lib/core/uri.dart
+++ b/sdk/lib/core/uri.dart
@@ -971,7 +971,7 @@ class Uri {
     }
     _checkNonWindowsPathReservedCharacters(pathSegments, false);
     var result = new StringBuffer();
-    if (isAbsolute) result.write("/");
+    if (_isPathAbsolute) result.write("/");
     result.writeAll(pathSegments, "/");
     return result.toString();
   }
@@ -989,7 +989,7 @@ class Uri {
       _checkWindowsPathReservedCharacters(segments, false);
     }
     var result = new StringBuffer();
-    if (isAbsolute && !hasDriveLetter) result.write("\\");
+    if (_isPathAbsolute && !hasDriveLetter) result.write("\\");
     if (host != "") {
       result.write("\\");
       result.write(host);
@@ -1000,6 +1000,11 @@ class Uri {
     return result.toString();
   }
 
+  bool get _isPathAbsolute {
+    if (path == null || path.isEmpty) return false;
+    return path.startsWith('/');
+  }
+
   void _writeAuthority(StringSink ss) {
     _addIfNonEmpty(ss, userInfo, userInfo, "@");
     ss.write(_host == null ? "null" : _host);