Fix function subtyping bug.

sdk/lib/_internal/lib/js_rti.dart

Index: sdk/lib/_internal/lib/js_rti.dart
diff --git a/sdk/lib/_internal/lib/js_rti.dart b/sdk/lib/_internal/lib/js_rti.dart
index d8feff58b943d0612dd801b591c1b78a749fcb52..d090dea84dc364d3fe516a582b27326e9e72f176 100644
--- a/sdk/lib/_internal/lib/js_rti.dart
+++ b/sdk/lib/_internal/lib/js_rti.dart
@@ -675,11 +675,11 @@ bool isFunctionSubtype(var s, var t) {
         return false;
       }
     }
-    sPos = 0;
-    // Check the optional parameters of [t] with the remaing optional parameters
-    // of [s]:
+    tPos = 0;
+    // Check the optional parameters of [t] with the remaining optional
+    // parameters of [s]:
     for (; tPos < tOptionalParametersLen ; sPos++, tPos++) {
-      if (!isAssignable(getIndex(tOptionalParameterTypes, sPos),
+      if (!isAssignable(getIndex(sOptionalParameterTypes, sPos),
                         getIndex(tOptionalParameterTypes, tPos))) {
         return false;
       }