Remove explicit bool type hack.

During the fix for https://github.com/dart-lang/language/issues/1785,
an explicit `bool` type was added to the variable `nullable` to ensure
that it would properly participate in type promotion while the fix was
still being rolled out.  Now that the fix is in place, this explicit
type is no longer needed.

TEST=standard trybots, TAP global presubmit, Golem
Change-Id: Ib81ad436876e576f85c929c205d5831214e6a05a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/214821
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

.dart_tool/package_config.json

@@ -11,7 +11,7 @@
     "constraint, update this by running tools/generate_package_config.dart."
   ],
   "configVersion": 2,
-  "generated": "2021-11-15T17:34:02.415038",
+  "generated": "2021-12-09T13:52:42.287379",
   "generator": "tools/generate_package_config.dart",
   "packages": [
     {
@@ -760,7 +760,7 @@
       "name": "vm",
       "rootUri": "../pkg/vm",
       "packageUri": "lib/",
-      "languageVersion": "2.12"
+      "languageVersion": "2.15"
     },
     {
       "name": "vm_service",