Allow trailing comma in parameter and argument lists.

runtime/vm/parser.cc

Index: runtime/vm/parser.cc
diff --git a/runtime/vm/parser.cc b/runtime/vm/parser.cc
index 7819cd780ac26f66bf7a1621d56cd8d5c8611bac..d4b3e6943137328308dceb84aec9ec922b3a9157 100644
--- a/runtime/vm/parser.cc
+++ b/runtime/vm/parser.cc
@@ -2031,6 +2031,14 @@ void Parser::ParseFormalParameters(bool allow_explicit_default_values,
       params->has_optional_named_parameters = true;
       return;
     }
+    Token::Kind terminator =
+       params->has_optional_positional_parameters ? Token::kRBRACK :
+       params->has_optional_named_parameters ? Token::kRBRACE :
+       Token :: kRPAREN;
+    if (CurrentToken() == terminator) {

[hausner, 2016/06/22 17:50:25]

You are allowing an empty list of optional positional or named arguments with
this. In other words, you would allow a formal parameter list like this:

foo(a, [])

Which is probably not what you intended. On the other hand, I don't have a
authoritative spec.

[Lasse Reichstein Nielsen, 2016/06/24 11:43:04]

Good point. 
Curiously, it's only an empty named block that gets accepted, something must be
catching the empty positional one elsewhere.

+      // Allow a trailing comma.
+      break;
+    }
     ParseFormalParameter(allow_explicit_default_values,
                          evaluate_metadata,
                          params);
@@ -11125,6 +11133,10 @@ ArgumentListNode* Parser::ParseActualParameters(
       ASSERT((CurrentToken() == Token::kLPAREN) ||
              (CurrentToken() == Token::kCOMMA));
       ConsumeToken();
+      if (CurrentToken() == Token::kRPAREN) {
+        // Allow trailing comma.
+        break;
+      }
       if (IsIdentifier() && (LookaheadToken(1) == Token::kCOLON)) {
         named_argument_seen = true;
         // The canonicalization of the arguments descriptor array built in