Fix dart2js test

sdk/lib/_internal/js_runtime/lib/io_patch.dart

Index: sdk/lib/_internal/js_runtime/lib/io_patch.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/io_patch.dart b/sdk/lib/_internal/js_runtime/lib/io_patch.dart
index 35d314fcd7d1676e030784d78014c1f1424dee8f..1b2f7d3cd9ce590dff01fe9093721afbe2af85a5 100644
--- a/sdk/lib/_internal/js_runtime/lib/io_patch.dart
+++ b/sdk/lib/_internal/js_runtime/lib/io_patch.dart
@@ -219,7 +219,7 @@ class _Platform {
     throw new UnsupportedError("Platform._version");
   }
   @patch
-  static _ansiSupported() {
+  static bool _ansiSupported() {
     throw new UnsupportedError("Platform._ansiSupported");
   }
 }