[vm/nnbd] Avoid passing null as a non-nullable argument of _AssertionError._create

In kernel files assertion conditions are specified as a range of source
positions. Sometimes source texts are not available and VM cannot
extract assertion condition text, so Script::GetSnippet returns null.

With null safety it is no longer valid to pass null as a non-nullable
argument '_failedAssertion' of _AssertionError._create constructor,
so we should provide a valid String. This change converts 'null'
assertion condition text to "<optimized out>", similarly to AOT mode.

Issue flutter/flutter#63513
Issue dart-lang/sdk#34586

Change-Id: I9c5791e98fdef358068f3f5ddc4cfe98e8c7ed36
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/158162
Reviewed-by: Régis Crelier <regis@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>

Author: alexmarkov

runtime/lib/errors.cc

@@ -90,10 +90,13 @@ DEFINE_NATIVE_ENTRY(AssertionError_throwNew, 0, 3) {
   script.GetTokenLocation(assertion_start, &from_line, &from_column);
   intptr_t to_line, to_column;
   script.GetTokenLocation(assertion_end, &to_line, &to_column);
-  // The snippet will extract the correct assertion code even if the source
-  // is generated.
-  args.SetAt(0, String::Handle(script.GetSnippet(from_line, from_column,
-                                                 to_line, to_column)));
+  // Extract the assertion condition text (if source is available).
+  auto& condition_text = String::Handle(
+      script.GetSnippet(from_line, from_column, to_line, to_column));
+  if (condition_text.IsNull()) {
+    condition_text = Symbols::OptimizedOut().raw();
+  }
+  args.SetAt(0, condition_text);
 
   // Initialize location arguments starting at position 1.
   // Do not set a column if the source has been generated as it will be wrong.