Fix parsing [a]sync[*] function body modifiers with Fasta.

pkg/front_end/lib/src/fasta/kernel/body_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.body_builder;
 
 import 'package:front_end/src/fasta/parser/parser.dart'
     show FormalParameterType, optional;
 
 import 'package:front_end/src/fasta/parser/error_kind.dart' show ErrorKind;
@@ skipping 823 matching lines @@
     push(new StringConcatenation(expressions ?? parts));
   }
 
   @override
   void handleLiteralInt(Token token) {
     debugEvent("LiteralInt");
     push(new IntLiteral(int.parse(token.value)));
   }
 
   @override
+  void endExpressionFunctionBody(Token arrowToken, Token endToken) {
+    debugEvent("ExpressionFunctionBody");
+    endReturnStatement(true, arrowToken, endToken);
+  }
+
+  @override
   void endReturnStatement(
       bool hasExpression, Token beginToken, Token endToken) {
     debugEvent("ReturnStatement");
     Expression expression = hasExpression ? popForValue() : null;
     if (expression != null && inConstructor) {
       push(buildCompileTimeErrorStatement(
           "Can't return from a constructor.", beginToken.charOffset));
     } else {
       push(new ReturnStatement(expression));
     }
@@ skipping 1818 matching lines @@
   } else if (node is TypeDeclarationBuilder) {
     return node.name;
   } else if (node is PrefixBuilder) {
     return node.name;
   } else if (node is ThisPropertyAccessor) {
     return node.name.name;
   } else {
     return internalError("Unhandled: ${node.runtimeType}");
   }
 }