[tfa] Use field type as type of temporary variable when removing fields in TFA

When a field gets never read TFA may remove the field entirely. The
initializer of the field may still need to be executed as it may have
side-effects. The initializer is then moved to a `LocalInitializer` with
a unused variable.

Currently this variable is typed `dynamic` but could be strenghtened to
be the type of the field, which this CL does.

The usage of `dynamic` can have problems for some backends that have
types that aren't part of the Dart object hierarchy (e.g. a
`WasmArray`). Those types cannot flow into top types as they are not
real Dart objects. By typing the `LocalInitializer` variable the same as
the field type, we avoid assigning such foreign, no-Dart objects to
variables of Dart top-type.

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

Change-Id: Idd4bf944afad83ceb1850f478e7a9d9cd47bf00d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/415201
Commit-Queue: Martin Kustermann <[email protected]>
Reviewed-by: Alexander Markov <[email protected]>

Author: mkustermann

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

@@ -1850,6 +1850,7 @@ class _TreeShakerPass1 extends RemovingTransformer {
               null,
               initializer: node.value,
               isSynthesized: true,
+              type: field.type,
             ),
           );
         } else {

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

@@ -23,13 +23,13 @@ class A extends core::Object {
   [@vm.unboxing-info.metadata=(i)->i]
   field core::int f7;
   constructor •() → self::A
-    : dynamic #t1 = self::sideEffect(1), dynamic #t2 = self::sideEffect(2), self::A::f7 = [@vm.inferred-type.metadata=dart.core::_Smi (value: 7)] self::sideEffect(7), dynamic #t3 = self::sideEffect(8), dynamic #t4 = [@vm.inferred-type.metadata=dart.core::_Smi (value: 100)] self::sideEffect(100), dynamic #t5 = [@vm.inferred-type.metadata=dart.core::_Smi (value: 200)] self::sideEffect(200), super core::Object::•()
+    : dynamic #t1 = self::sideEffect(1), dynamic #t2 = self::sideEffect(2), self::A::f7 = [@vm.inferred-type.metadata=dart.core::_Smi (value: 7)] self::sideEffect(7), dynamic #t3 = self::sideEffect(8), core::int #t4 = [@vm.inferred-type.metadata=dart.core::_Smi (value: 100)] self::sideEffect(100), core::int #t5 = [@vm.inferred-type.metadata=dart.core::_Smi (value: 200)] self::sideEffect(200), super core::Object::•()
     ;
   constructor foo() → self::A
     : this self::A::•()
     ;
   constructor bar() → self::A
-    : dynamic #t6 = self::sideEffect(1), dynamic #t7 = [@vm.inferred-type.metadata=dart.core::_Smi (value: 2)] self::sideEffect(2), self::A::f7 = [@vm.inferred-type.metadata=dart.core::_Smi (value: 7)] self::sideEffect(7), dynamic #t8 = self::sideEffect(8), dynamic #t9 = [@vm.inferred-type.metadata=dart.core::_Smi (value: 801)] self::sideEffect(801), super core::Object::•()
+    : dynamic #t6 = self::sideEffect(1), core::int #t7 = [@vm.inferred-type.metadata=dart.core::_Smi (value: 2)] self::sideEffect(2), self::A::f7 = [@vm.inferred-type.metadata=dart.core::_Smi (value: 7)] self::sideEffect(7), dynamic #t8 = self::sideEffect(8), core::int #t9 = [@vm.inferred-type.metadata=dart.core::_Smi (value: 801)] self::sideEffect(801), super core::Object::•()
     ;
 }
 

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

@@ -12,7 +12,7 @@ class B extends core::Object {
 }
 class C extends core::Object {
   synthetic constructor •() → self::C
-    : dynamic #t1 = new self::B::•(), super core::Object::•()
+    : self::B? #t1 = new self::B::•(), super core::Object::•()
     ;
 
   [@vm.procedure-attributes.metadata=methodOrSetterCalledDynamically:false,getterCalledDynamically:false,hasThisUses:false,hasTearOffUses:false,methodOrSetterSelectorId:1]