Add support for FieldFormalParameter.

pkg/front_end/lib/src/fasta/parser/parser.dart

Index: pkg/front_end/lib/src/fasta/parser/parser.dart
diff --git a/pkg/front_end/lib/src/fasta/parser/parser.dart b/pkg/front_end/lib/src/fasta/parser/parser.dart
index 19512820017d87d353339ceba1ae7bad30986160..0ea4a4a49bbe8622e6dc6300b4a00576f262c777 100644
--- a/pkg/front_end/lib/src/fasta/parser/parser.dart
+++ b/pkg/front_end/lib/src/fasta/parser/parser.dart
@@ -478,11 +478,13 @@ class Parser {
     // TODO(ahe): Validate that there are formal parameters if void.
     token = parseReturnTypeOpt(token);
     Token thisKeyword = null;
+    Token period = null;
     if (optional('this', token)) {
       thisKeyword = token;
+      period = token.next;

[Paul Berry, 2017/02/22 20:41:09]

This seems problematic for error recovery.  If the next token isn't a '.', we
should set period to null.

[scheglov, 2017/02/23 06:43:50]

This is the pattern that is used everywhere else in the parser.
See for example in parseDoWhileStatement().

Ideally of course we should recover just as we do in the Analyzer's
implementation, and create a synthetic token. But rewriting handling of invalid
cases and recovery seems out of scope of this CL. Might be a topic to discuss
tomorrow.

[ahe, 2017/02/23 16:27:11]

I agree that it is a problem for error recovery.

       // TODO(ahe): Validate field initializers are only used in
       // constructors, and not for function-typed arguments.
-      token = expect('.', token.next);
+      token = expect('.', period);
     }
     token = parseIdentifier(token);
     if (optional('(', token)) {
@@ -510,7 +512,7 @@ class Parser {
             equal, ErrorKind.PositionalParameterWithEquals);
       }
     }
-    listener.endFormalParameter(thisKeyword);
+    listener.endFormalParameter(thisKeyword, period);

[ahe, 2017/02/23 16:27:11]

This isn't necessary. The period can be found without guessing by looking at
thisKeyword.

     return token;
   }