fixes #40, extension methods for primitive types

test/codegen/expect/misc.js

 var misc = dart.defineLibrary(misc, {});
 var core = dart.import(core);
 (function(exports, core) {
   'use strict';
   class _Uninitialized extends core.Object {
     _Uninitialized() {
     }
   }
   dart.setSignature(_Uninitialized, {
     constructors: () => ({_Uninitialized: [_Uninitialized, []]})
   });
   let UNINITIALIZED = dart.const(new _Uninitialized());
   let Generic$ = dart.generic(function(T) {
     class Generic extends core.Object {
       get type() {
         return Generic$();
       }
     }
     return Generic;
   });
   let Generic = Generic$();
   function main() {
-    core.print(dart.toString(1));
-    core.print(dart.toString(1.0));
-    core.print(dart.toString(1.1));
+    core.print(dart[dartx.toString](1));
+    core.print(dart[dartx.toString](1.0));
+    core.print(dart[dartx.toString](1.1));
     let x = 42;
     core.print(dart.equals(x, core.Object));
     core.print(dart.equals(x, Generic));
     core.print(new (Generic$(core.int))().type);
   }
   dart.fn(main);
   // Exports:
   exports.UNINITIALIZED = UNINITIALIZED;
   exports.Generic$ = Generic$;
   exports.Generic = Generic;
   exports.main = main;
 })(misc, core);