Various semantic checks on formal parameters.

pkg/front_end/messages.yaml

 # Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 # Each entry in this map corresponds to a diagnostic message. Ideally, each
 # entry contains three parts:
 #
 # 1. A message template (template).
 #
 # 2. A suggestion for how to correct the problem (tip).
@@ skipping 388 matching lines @@
   template: "A type or modifier is required here."
   tip: "Try adding a type, 'var', 'const', or 'final'."
 
 AssertExtraneousArgument:
   template: "`assert` can't have more than two arguments."
   dart2jsCode: FASTA_FATAL
 
 AssertAsExpression:
   template: "`assert` can't be used as an expression."
   dart2jsCode: FASTA_FATAL
+
+FunctionTypeDefaultValue:
+  template: "Can't have a default value in a function type."
+  dart2jsCode: FASTA_IGNORED
+
+PrivateNamedParameter:
+  template: "An optional named parameter can't start with '_'."
+  dart2jsCode: FASTA_IGNORED

[danrubel, 2017/06/09 17:08:52]

If fasta recognizes these errors, then should they be reported by fasta and
removed or not reported by dart2js ?

[ahe, 2017/06/09 18:53:31]

The code "FASTA_IGNORED" doesn't mean that Fasta ignores the error.

What is dart2jsCode for? It's a code that we look up in MessageKind in
pkg/compiler/lib/src/diagnostics/messages.dart, and this way dart2js can
continue reporting the same errors that it has always done.

But sometimes we want to send a special signal to dart2js, for example, "please
ignore this error" (because it's already implemented in a different location in
dart2js).

But we have to be careful that what we write in this field so it doesn't
conflict with anything in MessageKind. So I figured, let's prefix all the
special codes with FASTA_.

I think this would also work, and be less confusing:

  dart2jsCode: "*ignored*"

and:

  dart2jsCode: "*fatal*"

WDYT?

So why not modify dart2js so that it doesn't double report errors that the
parser has already found? Well, the less changes I have to make to that part of
dart2js, the faster I can make progress on Fasta which should eventually replace
those parts of dart2js anyways.

Why not implement the check in BodyBuilder? Well, then I would also have to
implement it in AstBuilder. The more errors I can detect in the parser, the less
duplication between BodyBuilder and AstBuilder. This was early feedback from
Siggi and Paul: perhaps we should catch more errors in the parser.

+
+NoFormals:
+  template: "A function should have formal parameters."
+  tip: "Try adding '()' after '#lexeme', or add 'get' before '#lexeme' to declare a getter."
+  dart2jsCode: FASTA_IGNORED
+
+GetterWithFormals:
+  template: "A getter can't have formal parameters."
+  tip: "Try removing '(...)'."
+  dart2jsCode: FASTA_IGNORED
+
+CatchSyntax:
+  template: "'catch' must be followed by '(identifier)' or '(identifier, identifier)'."
+  tip: "No types are needed, the first is given by 'on', the second is always 'StackTrace'."
+  dart2jsCode: FASTA_IGNORED