Handle for-in loops

lib/runtime/dart/_js_names.js

 var _js_names;
 (function(exports) {
   'use strict';
   // Function preserveNames: () → dynamic
   function preserveNames() {
   }
   dart.defineLazyProperties(exports, {
     get mangledNames() {
       return computeMangledNames(_foreign_helper.JS_EMBEDDED_GLOBAL('=Object', _js_embedded_names.MANGLED_NAMES), false);
     },
     get reflectiveNames() {
       return computeReflectiveNames(exports.mangledNames);
     },
     get mangledGlobalNames() {
       return computeMangledNames(_foreign_helper.JS_EMBEDDED_GLOBAL('=Object', _js_embedded_names.MANGLED_GLOBAL_NAMES), true);
     },
     get reflectiveGlobalNames() {
       return computeReflectiveNames(exports.mangledGlobalNames);
     }
   });
   // Function computeMangledNames: (dynamic, bool) → Map<String, String>
   function computeMangledNames(jsMangledNames, isGlobal) {
     preserveNames();
     let keys = extractKeys(jsMangledNames);
     let result = dart.map();
     let getterPrefix = _foreign_helper.JS_GET_NAME('GETTER_PREFIX');
     let getterPrefixLength = getterPrefix.length;
     let setterPrefix = _foreign_helper.JS_GET_NAME('SETTER_PREFIX');
-    for (let key of keys) {
+    for (let key of dart.as(keys, core.Iterable$(core.String))) {
       let value = jsMangledNames[key];
       result.set(key, value);
       if (!dart.notNull(isGlobal)) {
         if (key.startsWith(getterPrefix)) {
           result.set(`${setterPrefix}${key.substring(getterPrefixLength)}`, `${value}=`);
         }
       }
     }
     return result;
   }
@@ skipping 30 matching lines @@
     });
   }
   // Exports:
   exports.preserveNames = preserveNames;
   exports.computeMangledNames = computeMangledNames;
   exports.computeReflectiveNames = computeReflectiveNames;
   exports.extractKeys = extractKeys;
   exports.unmangleGlobalNameIfPreservedAnyways = unmangleGlobalNameIfPreservedAnyways;
   exports.unmangleAllIdentifiersIfPreservedAnyways = unmangleAllIdentifiersIfPreservedAnyways;
 })(_js_names || (_js_names = {}));