[web-fixit] Avoid a few tests in production mode

Fixes:
language/control_flow_collections/await_for_test
Change-Id: I701c5455af4c5910c3cbaec06d1c75d35e87c597
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/338648
Reviewed-by: Bob Nystrom <rnystrom@google.com>
Commit-Queue: Stephen Adams <sra@google.com>
Reviewed-by: Mayank Patke <fishythefish@google.com>

tests/language/control_flow_collections/await_for_test.dart

@@ -281,6 +281,10 @@ Future<void> testKeyOrder() async {
 }
 
 Future<void> testRuntimeErrors() async {
+  // The checks for the TypeErrors tested in here are omitted in dart2js
+  // production mode.
+  if (dart2jsProductionMode) return;
+
   // Cast variable.
   dynamic nonStream = 3;
   asyncExpectThrows<TypeError>(() async {