fix some errors in our SDK, mostly around numbers

lib/runtime/dart/convert.js

 dart_library.library('dart/convert', null, /* Imports */[
   "dart_runtime/dart",
   'dart/core',
   'dart/async',
   'dart/typed_data',
   'dart/_internal',
   'dart/collection'
 ], /* Lazy imports */[
 ], function(exports, dart, core, async, typed_data, _internal, collection) {
   'use strict';
@@ skipping 1188 matching lines @@
         if (!dart.notNull(this.writeJsonValue(customJson))) {
           dart.throw(new JsonUnsupportedObjectError(object));
         }
         this[_removeSeen](object);
       } catch (e) {
         dart.throw(new JsonUnsupportedObjectError(object, {cause: e}));
       }
 
     }
     writeJsonValue(object) {
-      if (dart.is(object, core.num)) {
+      if (typeof object == 'number') {
         if (!dart.notNull(dart.as(dart.dload(object, 'isFinite'), core.bool)))
           return false;
         this.writeNumber(dart.as(object, core.num));
         return true;
       } else if (dart.notNull(core.identical(object, true))) {
         this.writeString('true');
         return true;
       } else if (dart.notNull(core.identical(object, false))) {
         this.writeString('false');
         return true;
@@ skipping 1675 matching lines @@
   exports.LineSplitter = LineSplitter;
   exports.StringConversionSink = StringConversionSink;
   exports.ClosableStringSink = ClosableStringSink;
   exports.UNICODE_REPLACEMENT_CHARACTER_RUNE = UNICODE_REPLACEMENT_CHARACTER_RUNE;
   exports.UNICODE_BOM_CHARACTER_RUNE = UNICODE_BOM_CHARACTER_RUNE;
   exports.Utf8Codec = Utf8Codec;
   exports.UTF8 = UTF8;
   exports.Utf8Encoder = Utf8Encoder;
   exports.Utf8Decoder = Utf8Decoder;
 });