Work around changes to Platform in docgen

pkg/docgen/lib/docgen.dart

Index: pkg/docgen/lib/docgen.dart
diff --git a/pkg/docgen/lib/docgen.dart b/pkg/docgen/lib/docgen.dart
index 1d147c700b939ff33238a2477ac1a655d24d329c..7f4dd9cde1eb0d7d70934aca6c135f7a1dbabe8d 100644
--- a/pkg/docgen/lib/docgen.dart
+++ b/pkg/docgen/lib/docgen.dart
@@ -249,7 +249,9 @@ Future<MirrorSystem> getMirrorSystem(List<String> args, {String packageRoot,
   var libraries = !parseSdk ? _listLibraries(args) : _listSdk();
   if (libraries.isEmpty) throw new StateError('No Libraries.');
   // Finds the root of SDK library based off the location of docgen.
-  var sdkRoot = Platform.script.resolve('../../../sdk').toFilePath();
+  var scriptDir = path.absolute(path.dirname(Platform.script.toFilePath()));
+  var sdkRoot = path.relative('../../../sdk', from: scriptDir);
+  sdkRoot = path.normalize(path.absolute(sdkRoot));
   logger.info('SDK Root: ${sdkRoot}');
   return _analyzeLibraries(libraries, sdkRoot, packageRoot: packageRoot);
 }
@@ -478,9 +480,10 @@ void _mdnComment(Indexable item) {
   //Check if MDN is loaded.
   if (_mdn == null) {
     // Reading in MDN related json file.
-    var mdnDatabase =
-        Platform.script.resolve('../../../utils/apidoc/mdn/database.json');
-    _mdn = JSON.decode(new File(mdnDatabase.toFilePath()).readAsStringSync());
+    var scriptDir = path.absolute(path.dirname(Platform.script.toFilePath()));
+    var mdnPath = path.relative('../../../utils/apidoc/mdn/database.json',
+        from: scriptDir);
+    _mdn = JSON.decode(new File(mdnPath).readAsStringSync());
   }
   if (item.comment.isNotEmpty) return;
   var domAnnotation = item.annotations.firstWhere(