fix some errors in our SDK, mostly around numbers

lib/runtime/dart/_js_helper.js

Index: lib/runtime/dart/_js_helper.js
diff --git a/lib/runtime/dart/_js_helper.js b/lib/runtime/dart/_js_helper.js
index 066745949bb2ba3049a789e81debfd28ae1eb142..a7fe689043318fc96c22da267e9f358d51cdda15 100644
--- a/lib/runtime/dart/_js_helper.js
+++ b/lib/runtime/dart/_js_helper.js
@@ -822,13 +822,13 @@ dart_library.library('dart/_js_helper', null, /* Imports */[
       return value;
     }
     static getProperty(object, key) {
-      if (object == null || typeof object == 'boolean' || dart.is(object, core.num) || typeof object == 'string') {
+      if (object == null || typeof object == 'boolean' || typeof object == 'number' || typeof object == 'string') {
         dart.throw(new core.ArgumentError(object));
       }
       return object[key];
     }
     static setProperty(object, key, value) {
-      if (object == null || typeof object == 'boolean' || dart.is(object, core.num) || typeof object == 'string') {
+      if (object == null || typeof object == 'boolean' || typeof object == 'number' || typeof object == 'string') {
         dart.throw(new core.ArgumentError(object));
       }
       object[key] = value;
@@ -895,7 +895,7 @@ dart_library.library('dart/_js_helper', null, /* Imports */[
   }
   dart.fn(checkNull);
   function checkNum(value) {
-    if (!dart.is(value, core.num)) {
+    if (!(typeof value == 'number')) {
       dart.throw(new core.ArgumentError(value));
     }
     return value;