[dart:js_interop] Make isUndefined and isNull throw on dart2wasm

null and undefined cannot be distinguished on dart2wasm in its
current state, so these helpers should only work on the JS
compilers. Some comments are updated to reflect the current state
of this internalization. Also fixes a pending TODO in isNull and
isUndefined on the JS backends.

CoreLibraryReviewExempt: Backend-specific library.
Change-Id: Ic56e8aa346af99cb99d01fe3c7ac5e37e965db23
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/326690
Commit-Queue: Srujan Gaddam <srujzs@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>

https://dart.googlesource.com/sdk/+/e130bb36cee85c3466d79b9eaf39cb572e17c81a

DEPS

diff --git a/DEPS b/DEPS
index 033c72f..9c986e5 100644
--- a/DEPS
+++ b/DEPS
@@ -267,7 +267,7 @@
 
   'engine/src/flutter': Var('flutter_git') + '/mirrors/engine' + '@' + 'f9de0665e7def81fb9c8614d8043fe2fa6bc5f4b',
 
-  'engine/src/third_party/dart': Var('dart_git') + '/sdk' + '@' + 'e3852368b5d17cf42794b97552e76347266f54fd',
+  'engine/src/third_party/dart': Var('dart_git') + '/sdk' + '@' + 'e130bb36cee85c3466d79b9eaf39cb572e17c81a',
 
   'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '6379648b7241631a665b7660e1b1e7421af8ad93',
 

commits.json

diff --git a/commits.json b/commits.json
index c2026ed..d52521a 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "engine/src/flutter":"f9de0665e7def81fb9c8614d8043fe2fa6bc5f4b",
-  "engine/src/third_party/dart":"e3852368b5d17cf42794b97552e76347266f54fd",
+  "engine/src/third_party/dart":"e130bb36cee85c3466d79b9eaf39cb572e17c81a",
   "flutter":"6379648b7241631a665b7660e1b1e7421af8ad93"
 }
\ No newline at end of file