Revert "Remove explicit bool type hack."

This reverts commit 4a54307a39.

Reason for revert: Broke internal build, golem

Original change's description:
> 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
> Change-Id: Ied468b17dafa03075fd54c0df915e0a539420697
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/212266
> Reviewed-by: Alexander Markov <alexmarkov@google.com>
> Commit-Queue: Paul Berry <paulberry@google.com>

TBR=paulberry@google.com,alexmarkov@google.com

Change-Id: I748060275968e1a87a78e90d3770b17543549f33
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/212783
Reviewed-by: Paul Berry <paulberry@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": "2021-09-07T14:56:54.026010",
+  "generated": "2021-09-01T12:41:37.776425",
   "generator": "tools/generate_package_config.dart",
   "packages": [
     {
@@ -761,7 +761,7 @@
       "name": "vm",
       "rootUri": "../pkg/vm",
       "packageUri": "lib/",
-      "languageVersion": "2.15"
+      "languageVersion": "2.12"
     },
     {
       "name": "vm_service",

pkg/vm/lib/transformations/type_flow/transformer.dart

@@ -315,7 +315,13 @@ class AnnotateKernel extends RecursiveVisitor {
     Constant? constantValue;
     bool isInt = false;
 
-    final nullable = type is NullableType;
+    // Note: the explicit type `bool` is needed because the checked-in version
+    // of the CFE that we use for bootstrapping doesn't yet have constructor
+    // tearoffs enabled, and the fix for bug
+    // https://github.com/dart-lang/language/issues/1785 only takes effect when
+    // constructor tearoffs are enabled.  TODO(paulberry): remove the type after
+    // the bootstrap CFE enables constructor tearoffs.
+    final bool nullable = type is NullableType;
     if (nullable) {
       type = type.baseType;
     }

pkg/vm/pubspec.yaml

@@ -4,7 +4,7 @@ description: VM specific Dart code and helper scripts
 publish_to: none
 
 environment:
-  sdk: '>=2.15.0 <3.0.0'
+  sdk: '>=2.12.0 <3.0.0'
 
 dependencies:
   args: ^1.4.4