[vm] Implementation of the null check instruction in the flow graph.

tests/language_2/nnbd/syntax/non_null_assertion_test.dart now passes

Bug: #38840
Change-Id: Ie9676583c2cd7434dbb1c4c8b5ee4db817d01228
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/121820
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Commit-Queue: Liam Appelbe <liama@google.com>

Author: liamappelbe

runtime/vm/compiler/frontend/kernel_binary_flowgraph.cc

@@ -3325,9 +3325,11 @@ Fragment StreamingFlowGraphBuilder::BuildNullCheck(TokenPosition* p) {
   if (p != nullptr) *p = position;
 
   TokenPosition operand_position = TokenPosition::kNoSource;
-  Fragment instructions =
-      BuildExpression(&operand_position);  // read expression.
-  // TODO(37479): Implement null-check semantics.
+  Fragment instructions = BuildExpression(&operand_position);
+  LocalVariable* expr_temp = MakeTemporary();
+  instructions += CheckNull(position, expr_temp, String::null_string(),
+                            /* clear_the_temp = */ false);
+
   return instructions;
 }
 

runtime/vm/runtime_entry.cc

@@ -153,6 +153,17 @@ DEFINE_RUNTIME_ENTRY(RangeError, 2) {
 }
 
 static void NullErrorHelper(Zone* zone, const String& selector) {
+  // If the selector is null, this must be a null check that wasn't due to a
+  // method invocation, so was due to the null check operator.
+  if (selector.IsNull()) {
+    const Array& args = Array::Handle(zone, Array::New(4));
+    args.SetAt(
+        3, String::Handle(
+               zone, String::New("Null check operator used on a null value")));
+    Exceptions::ThrowByType(Exceptions::kCast, args);
+    return;
+  }
+
   InvocationMirror::Kind kind = InvocationMirror::kMethod;
   if (Field::IsGetterName(selector)) {
     kind = InvocationMirror::kGetter;