Source positions for constructors and lots of async machinery

runtime/vm/scanner.h

Index: runtime/vm/scanner.h
diff --git a/runtime/vm/scanner.h b/runtime/vm/scanner.h
index 7a8104081f3408be2f712ef00d0fdac7eeb1f3dc..38748557eca92532fa5902d472f7e69864857b88 100644
--- a/runtime/vm/scanner.h
+++ b/runtime/vm/scanner.h
@@ -45,7 +45,7 @@ class Scanner : ValueObject {
   };
 
   // Dummy token index reflecting an unknown source position.
-  static const intptr_t kNoSourcePos = -1;
+  static const intptr_t kNoSourcePos = Token::kNoSourcePos;
 
   static bool ValidSourcePosition(intptr_t token_pos) {
     return (token_pos >= 0) || (token_pos == kNoSourcePos);