fixes #40, extension methods for primitive types

lib/runtime/dart/_js_names.js

 var _js_names = dart.defineLibrary(_js_names, {});
 var _foreign_helper = dart.lazyImport(_foreign_helper);
 var _js_embedded_names = dart.import(_js_embedded_names);
 var core = dart.import(core);
 var _interceptors = dart.import(_interceptors);
 var _js_helper = dart.lazyImport(_js_helper);
 (function(exports, _foreign_helper, _js_embedded_names, core, _interceptors, _js_helper) {
   'use strict';
   function preserveNames() {
   }
@@ skipping 16 matching lines @@
     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 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}=`);
+        if (key[dartx.startsWith](getterPrefix)) {
+          result.set(`${setterPrefix}${key[dartx.substring](getterPrefixLength)}`, `${value}=`);
         }
       }
     }
     return result;
   }
   dart.fn(computeMangledNames, core.Map$(core.String, core.String), [core.Object, core.bool]);
   function computeReflectiveNames(map) {
     preserveNames();
     let result = dart.map();
     map.forEach(dart.fn((mangledName, reflectiveName) => {
@@ skipping 26 matching lines @@
   }
   dart.fn(unmangleAllIdentifiersIfPreservedAnyways, core.String, [core.String]);
   // Exports:
   exports.preserveNames = preserveNames;
   exports.computeMangledNames = computeMangledNames;
   exports.computeReflectiveNames = computeReflectiveNames;
   exports.extractKeys = extractKeys;
   exports.unmangleGlobalNameIfPreservedAnyways = unmangleGlobalNameIfPreservedAnyways;
   exports.unmangleAllIdentifiersIfPreservedAnyways = unmangleAllIdentifiersIfPreservedAnyways;
 })(_js_names, _foreign_helper, _js_embedded_names, core, _interceptors, _js_helper);