[vm/nnbd] Fix CompileType::IsAssignableTo for nullable types in strong mode

Also, tests/corelib/string_operations_with_null_test.dart is fixed for
strong mode: in strong mode TypeError is thrown when null is casted
to a non-nullable type; in weak mode null is silently passed and
ArgumentError or NoSuchMethodError is thrown.

Change-Id: I566a80293ffb03752cc3409e1b79491f0aff6888
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/140779
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>

Author: alexmarkov

runtime/vm/compiler/backend/type_propagator.cc

@@ -815,17 +815,13 @@ bool CompileType::IsAssignableTo(const AbstractType& other) {
   if (other.IsTopTypeForAssignability()) {
     return true;
   }
-
   if (IsNone()) {
     return false;
   }
-
-  // Consider the compile type of the value.
-  const AbstractType& compile_type = *ToAbstractType();
-  if (compile_type.IsNullType()) {
-    return Instance::NullIsAssignableTo(other);
+  if (is_nullable() && !Instance::NullIsAssignableTo(other)) {
+    return false;
   }
-  return compile_type.IsSubtypeOf(other, Heap::kOld);
+  return ToAbstractType()->IsSubtypeOf(other, Heap::kOld);
 }
 
 bool CompileType::IsInstanceOf(const AbstractType& other) {

tests/corelib/string_operations_with_null_test.dart

@@ -11,7 +11,8 @@ returnStringOrNull() {
 }
 
 main() {
-  Expect.throwsArgumentError(() => 'foo' + returnStringOrNull());
+  Expect.throws(() => 'foo' + returnStringOrNull(),
+      (e) => e is ArgumentError || e is TypeError);
   Expect.throws(() => 'foo'.split(returnStringOrNull()),
-      (e) => e is ArgumentError || e is NoSuchMethodError);
+      (e) => e is ArgumentError || e is NoSuchMethodError || e is TypeError);
 }