[vm/aot/tfa] Avoid using null as an initial value of non-nullable and late variables without initializer

TEST=pkg/vm/testcases/transformations/type_flow/summary_collector/vars.dart

Cq-Include-Trybots: luci.dart.try:vm-kernel-precomp-nnbd-linux-release-x64-try
Change-Id: Ie6b3bdf7069e7d6389e3a729f23b9e1912a4eed2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/285061
Reviewed-by: Slava Egorov <[email protected]>
Commit-Queue: Alexander Markov <[email protected]>

Author: alexmarkov

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

@@ -2253,8 +2253,11 @@ class SummaryCollector extends RecursiveResultVisitor<TypeExpr?> {
   TypeExpr? visitVariableDeclaration(VariableDeclaration node) {
     node.annotations.forEach(_visit);
     final initializer = node.initializer;
-    final TypeExpr initialValue =
-        initializer == null ? _nullType : _visit(initializer);
+    final TypeExpr initialValue = initializer == null
+        ? ((node.type.nullability == Nullability.nonNullable || node.isLate)
+            ? const EmptyType()
+            : _nullType)
+        : _visit(initializer);
     _declareVariable(node, initialValue);
     return null;
   }

pkg/vm/testcases/transformations/type_flow/summary_collector/vars.dart

@@ -40,4 +40,22 @@ int loop2(int x) {
   return x;
 }
 
+int nonNullableWithoutInitializer(bool cond) {
+  int x;
+  if (cond) {
+    x = 5;
+  } else if (true) {
+    x = 7;
+  }
+  return x;
+}
+
+int? lateWithoutInitializer(bool cond) {
+  late int? x;
+  if (cond) {
+    x = 5;
+  }
+  return x;
+}
+
 main() {}

pkg/vm/testcases/transformations/type_flow/summary_collector/vars.dart.expect

@@ -40,6 +40,13 @@ t3* = _Call [dart.core::num.<] (i_0, _T (dart.core::_Smi, 5))
 t4* = _Call [dart.core::num.+] (_T (dart.core::int)+?, _T (dart.core::_Smi, 10))
 x_0 = _Join [dart.core::int] (%x, t4)
 RESULT: x_0
+------------ nonNullableWithoutInitializer ------------
+%cond = _Parameter #0 [_T (dart.core::bool)+?]
+x_0 = _Join [dart.core::int] (_T (dart.core::_Smi, 5), _T (dart.core::_Smi, 7))
+RESULT: x_0
+------------ lateWithoutInitializer ------------
+%cond = _Parameter #0 [_T (dart.core::bool)+?]
+RESULT: _T (dart.core::_Smi, 5)
 ------------ main ------------
 
 RESULT: _T {}?