Source positions for constructors and lots of async machinery

runtime/vm/ast.h

Index: runtime/vm/ast.h
diff --git a/runtime/vm/ast.h b/runtime/vm/ast.h
index 01ae0cf1544a072769676c3d7e6b402ea7186033..7d12d69e139b131af0e18d908d873e9276aac584 100644
--- a/runtime/vm/ast.h
+++ b/runtime/vm/ast.h
@@ -99,7 +99,8 @@ class AstNode : public ZoneAllocated {
  public:
   explicit AstNode(intptr_t token_pos)
       : token_pos_(token_pos) {
-    ASSERT(Scanner::ValidSourcePosition(token_pos_));
+    ASSERT(Scanner::ValidSourcePosition(token_pos_) ||
+           (token_pos_ == ClassifyingTokenPositions::kMethodExtractor));
   }
   virtual ~AstNode() { }