fix a few more codegen issues:

lib/runtime/_classes.js

Index: lib/runtime/_classes.js
diff --git a/lib/runtime/_classes.js b/lib/runtime/_classes.js
index 4497fb7aa54ea81e176ee38b2d3f7f90363f7a8d..9487e077b699893b4b3e8a324cf79d3a0be72cfc 100644
--- a/lib/runtime/_classes.js
+++ b/lib/runtime/_classes.js
@@ -393,6 +393,10 @@ dart_library.library('dart_runtime/_classes', null, /* Imports */[
 
   function canonicalMember(obj, name) {
     if (obj != null && obj[_extensionType]) return dartx[name];
+    // Check for certain names that we can't use in JS
+    if (name == 'constructor' || name == 'prototype') {
+      name = '+' + name;
+    }
     return name;
   }
   exports.canonicalMember = canonicalMember;