Don't use type parameters for types that come from JS

JS values never have reified generics.

Author: nex3

lib/src/node/render_options.dart

@@ -13,7 +13,7 @@ class RenderOptions {
   external String get data;
   external dynamic get importer;
   external dynamic get functions;
-  external List<String> get includePaths;
+  external List get includePaths; // contains Strings
   external bool get indentedSyntax;
   external bool get omitSourceMapUrl;
   external String get outFile;

lib/src/node/render_result.dart

@@ -24,7 +24,7 @@ class RenderResultStats {
   external int get start;
   external int get end;
   external int get duration;
-  external List<String> get includedFiles;
+  external List get includedFiles; // contains Strings
 
   external factory RenderResultStats(
       {String entry,