allow JS builtin to be typed as needed

lib/runtime/dart/convert.js

Index: lib/runtime/dart/convert.js
diff --git a/lib/runtime/dart/convert.js b/lib/runtime/dart/convert.js
index 0b4fababe82c6b70976c9030b3c076958fe62e66..43799a22be9d9ef61f02a5fdb8a0d125509a2fbd 100644
--- a/lib/runtime/dart/convert.js
+++ b/lib/runtime/dart/convert.js
@@ -2657,7 +2657,7 @@ dart_library.library('dart/convert', null, /* Imports */[
     }
     get [_upgradedMap]() {
       dart.assert(this[_isUpgraded]);
-      return dart.as(this[_data], core.Map);
+      return this[_data];
     }
     [_computeKeys]() {
       dart.assert(!dart.notNull(this[_isUpgraded]));
@@ -2665,7 +2665,7 @@ dart_library.library('dart/convert', null, /* Imports */[
       if (keys == null) {
         keys = this[_data] = _JsonMap._getPropertyNames(this[_original]);
       }
-      return dart.as(keys, core.List$(core.String));
+      return keys;
     }
     [_upgrade]() {
       if (dart.notNull(this[_isUpgraded])) return this[_upgradedMap];
@@ -2700,7 +2700,7 @@ dart_library.library('dart/convert', null, /* Imports */[
       return object[key] = value;
     }
     static _getPropertyNames(object) {
-      return dart.as(Object.keys(object), core.List);
+      return Object.keys(object);
     }
     static _isUnprocessed(object) {
       return typeof object == "undefined";