Change return type of Fangorn.parenthesizedCondition

Change-Id: Ie023607a0a85adcb710ec91174188a2013146254
Reviewed-on: https://dart-review.googlesource.com/56100
Reviewed-by: Dmitry Stefantsov <dmitryas@google.com>
Commit-Queue: Peter von der Ahé <ahe@google.com>

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

@@ -304,7 +304,7 @@ class Fangorn extends Forest<Expression, Statement, Token, Arguments> {
   }
 
   @override
-  Object parenthesizedCondition(
+  Expression parenthesizedCondition(
       Token leftParenthesis, Expression expression, Token rightParenthesis) {
     return expression;
   }