fix some errors in our SDK, mostly around numbers

lib/runtime/dart/_js_helper.js

 dart_library.library('dart/_js_helper', null, /* Imports */[
   "dart_runtime/dart",
   'dart/core',
   'dart/collection',
   'dart/_interceptors',
   'dart/_foreign_helper'
 ], /* Lazy imports */[
 ], function(exports, dart, core, collection, _interceptors, _foreign_helper) {
   'use strict';
   let dartx = dart.dartx;
@@ skipping 804 matching lines @@
     }
     static valueFromDateString(str) {
       if (!(typeof str == 'string'))
         dart.throw(new core.ArgumentError(str));
       let value = Date.parse(str);
       if (dart.notNull(value[dartx.isNaN]))
         dart.throw(new core.ArgumentError(str));
       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;
     }
     static identicalImplementation(a, b) {
       return a == null ? b == null : a === b;
     }
     static extractStackTrace(error) {
       return getTraceFromException(error.$thrownJsError);
     }
@@ skipping 46 matching lines @@
     return receiver.lastIndexOf(element, start);
   }
   dart.fn(stringLastIndexOfUnchecked);
   function checkNull(object) {
     if (object == null)
       dart.throw(new core.ArgumentError(null));
     return object;
   }
   dart.fn(checkNull);
   function checkNum(value) {
-    if (!dart.is(value, core.num)) {
+    if (!(typeof value == 'number')) {
       dart.throw(new core.ArgumentError(value));
     }
     return value;
   }
   dart.fn(checkNum);
   function checkInt(value) {
     if (!(typeof value == 'number')) {
       dart.throw(new core.ArgumentError(value));
     }
     return value;
@@ skipping 333 matching lines @@
   exports.CastErrorImplementation = CastErrorImplementation;
   exports.FallThroughErrorImplementation = FallThroughErrorImplementation;
   exports.RuntimeError = RuntimeError;
   exports.random64 = random64;
   exports.jsonEncodeNative = jsonEncodeNative;
   exports.SyncIterator$ = SyncIterator$;
   exports.SyncIterator = SyncIterator;
   exports.SyncIterable$ = SyncIterable$;
   exports.SyncIterable = SyncIterable;
 });