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

pkg/front_end/lib/src/fasta/kernel/body_builder.dart

Index: pkg/front_end/lib/src/fasta/kernel/body_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/body_builder.dart b/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
index b1d82d8e3a1a0915b191d990f61eff41a45e05a2..cae9a1039a640bcf70878c5df8c726f020e4242d 100644
--- a/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
@@ -841,6 +841,12 @@ class BodyBuilder extends ScopeListener<JumpTarget> implements BuilderHelper {
   }
 
   @override
+  void endExpressionFunctionBody(Token arrowToken, Token endToken) {
+    debugEvent("ExpressionFunctionBody");
+    endReturnStatement(true, arrowToken, endToken);
+  }
+
+  @override
   void endReturnStatement(
       bool hasExpression, Token beginToken, Token endToken) {
     debugEvent("ReturnStatement");