Add support for FieldFormalParameter.

pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart

 // Copyright (c) 2016, 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.
 
 library fasta.analyzer.ast_builder;
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/dart/ast/ast_factory.dart' show AstFactory;
 import 'package:analyzer/dart/ast/standard_ast_factory.dart' as standard;
 import 'package:analyzer/dart/ast/token.dart' as analyzer show Token;
@@ skipping 454 matching lines @@
         thenExpression, toAnalyzerToken(colon), elseExpression));
   }
 
   void endThrowExpression(Token throwToken, Token endToken) {
     debugEvent("ThrowExpression");
     push(ast.throwExpression(toAnalyzerToken(throwToken), pop()));
   }
 
   void endFormalParameter(Token thisKeyword) {
     debugEvent("FormalParameter");
-    if (thisKeyword != null) {
-      internalError("'this' can't be used here.");
-    }
     SimpleIdentifier name = pop();
     TypeName type = pop();
     Token keyword = _popOptionalSingleModifier();
     pop(); // TODO(paulberry): Metadata.
     // TODO(paulberry): handle covariant keyword.
-    SimpleFormalParameter node = ast.simpleFormalParameter(
-        null, null, toAnalyzerToken(keyword), type, name);
+
+    FormalParameter node;
+    if (thisKeyword == null) {
+      node = ast.simpleFormalParameter(
+          null, null, toAnalyzerToken(keyword), type, name);
+    } else {
+      // TODO(scheglov): Ideally the period token should be passed in.
+      Token period = identical('.', thisKeyword.next?.stringValue)
+          ? thisKeyword.next
+          : null;
+      TypeParameterList typeParameters; // TODO(scheglov)
+      FormalParameterList formalParameters; // TODO(scheglov)
+      node = ast.fieldFormalParameter(
+          null,
+          null,
+          toAnalyzerToken(keyword),
+          type,
+          toAnalyzerToken(thisKeyword),
+          toAnalyzerToken(period),
+          name,
+          typeParameters,
+          formalParameters);
+    }
+
     scope[name.name] = name.staticElement = new AnalyzerParameterElement(node);
     push(node);
   }
 
   void endFormalParameters(int count, Token beginToken, Token endToken) {
     debugEvent("FormalParameters");
     List<FormalParameter> parameters = popList(count) ?? <FormalParameter>[];
     push(ast.formalParameterList(toAnalyzerToken(beginToken), parameters, null,
         null, toAnalyzerToken(endToken)));
   }
@@ skipping 688 matching lines @@
 /// [ClassDeclaration] or [ClassTypeAlias] object.
 class _MixinApplication {
   final TypeName supertype;
 
   final Token withKeyword;
 
   final List<TypeName> mixinTypes;
 
   _MixinApplication(this.supertype, this.withKeyword, this.mixinTypes);
 }