[vm,bytecode] Correct assertion for source positions of yield points

Front-end sometimes generates synthetic awaits (yield points) which
don't have a source position. This change fixes assertion in bytecode
generator to account for that.

Issue: #38979
Change-Id: I64ded6a2e3bb9678ac3a7ddc73b117bdd52c3305
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/122177
Reviewed-by: Aart Bik <ajcbik@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>

Author: alexmarkov

pkg/vm/lib/bytecode/source_positions.dart

@@ -40,7 +40,7 @@ class SourcePositions {
 
   void addYieldPoint(int pc, int fileOffset) {
     assert(pc > _lastPc);
-    assert(fileOffset >= 0);
+    assert((fileOffset >= 0) || (fileOffset == syntheticCodeMarker));
     _positions.add(pc);
     _positions.add(yieldPointMarker);
     _positions.add(pc);