[tfa] Fix bug in TFA transformation of unused fields

When a TFA eliminates a field it moves it's initializer to a
`LocalInitializer` of the constructor. After [0] the
`VariableDeclaration` of the local initializer will get the type of the
field.

Though that CL didn't ensure TFA also visits the type of the
`VariableDeclaration`. This can lead to a situation where the type
refers to a class that TFA has tree shaken, which causes dangling
references in the AST.

[0] https://dart-review.googlesource.com/c/sdk/+/415201

Issue flutter/flutter#166124
Issue b/406692736

TEST=pkg/vm/testcases/transformations/type_flow/transformer/write_only_field

Change-Id: Ieb8f5c4daedd314c613617218da898059bb6941a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/419542
Reviewed-by: Slava Egorov <[email protected]>
Commit-Queue: Martin Kustermann <[email protected]>

Author: mkustermann

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

@@ -1850,7 +1850,7 @@ class _TreeShakerPass1 extends RemovingTransformer {
               null,
               initializer: node.value,
               isSynthesized: true,
-              type: field.type,
+              type: visitDartType(field.type, cannotRemoveSentinel),
             ),
           );
         } else {

pkg/vm/testcases/transformations/type_flow/transformer/write_only_field.dart

@@ -21,8 +21,11 @@ class B {
 
 class C {
   B? instanceField = new B();
+  UsedAsType<int>? Function() instanceField2 = () => null;
 }
 
+class UsedAsType<T> {}
+
 void main() {
   field = null;
   new C().instanceField = null;

pkg/vm/testcases/transformations/type_flow/transformer/write_only_field.dart.expect

@@ -12,12 +12,14 @@ class B extends core::Object {
 }
 class C extends core::Object {
   synthetic constructor •() → self::C
-    : self::B? #t1 = new self::B::•(), super core::Object::•()
+    : self::B? #t1 = new self::B::•(), () → self::UsedAsType<core::int>? #t2 = () → Null => null, super core::Object::•()
     ;
 
   [@vm.procedure-attributes.metadata=methodOrSetterCalledDynamically:false,getterCalledDynamically:false,hasThisUses:false,hasTearOffUses:false,methodOrSetterSelectorId:1]
   set instanceField(synthesized self::B? value) → void;
 }
+abstract class UsedAsType<T extends core::Object? = dynamic> extends core::Object {
+}
 
 [@vm.inferred-return-type.metadata=dart.core::Null? (value: null)]
 static method main() → void {