fixes #202, check IndexExpression

lib/runtime/dart/_internal.js

Index: lib/runtime/dart/_internal.js
diff --git a/lib/runtime/dart/_internal.js b/lib/runtime/dart/_internal.js
index db17b3fb4a7939afae902108086de0cc1b16acfb..7e5da986d2951177a944b131dafcecc151d6c853 100644
--- a/lib/runtime/dart/_internal.js
+++ b/lib/runtime/dart/_internal.js
@@ -1721,7 +1721,7 @@ var _js_primitives = dart.lazyImport(_js_primitives);
         this[_values] = values;
       }
       get(key) {
-        return this.containsKey(key) ? this[_values][core.$get](key) : null;
+        return this.containsKey(key) ? this[_values][core.$get](dart.as(key, core.int)) : null;
       }
       get length() {
         return this[_values][core.$length];