Remove explicit bool type hack.

During the fix for dart-lang/language#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 <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

.dart_tool/package_config.json

@@ -11,7 +11,7 @@
     "constraint, update this by running tools/generate_package_config.dart."
   ],
   "configVersion": 2,
-  "generated": "2021-09-01T12:41:37.776425",
+  "generated": "2021-09-07T14:56:54.026010",
   "generator": "tools/generate_package_config.dart",
   "packages": [
     {
@@ -761,7 +761,7 @@
       "name": "vm",
       "rootUri": "../pkg/vm",
       "packageUri": "lib/",
-      "languageVersion": "2.12"
+      "languageVersion": "2.15"
     },
     {
       "name": "vm_service",

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

@@ -315,13 +315,7 @@ class AnnotateKernel extends RecursiveVisitor {
     Constant? constantValue;
     bool isInt = false;
 
-    // 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;
+    final 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: '>=2.12.0 <3.0.0'
+  sdk: '>=2.15.0 <3.0.0'
 
 dependencies:
   args: ^1.4.4