compiler: disallow fallthrough in last case of switch.

In spec: A "fallthrough" statement may appear as the last
statement of all but the last clause of an expression switch.

From-SVN: r200440

gcc/go/gofrontend/parse.cc

@@ -4523,9 +4523,12 @@ Parse::expr_case_clause(Case_clauses* clauses, bool* saw_default)
   bool is_fallthrough = false;
   if (this->peek_token()->is_keyword(KEYWORD_FALLTHROUGH))
     {
+      Location fallthrough_loc = this->location();
       is_fallthrough = true;
       if (this->advance_token()->is_op(OPERATOR_SEMICOLON))
 	this->advance_token();
+      if (this->peek_token()->is_op(OPERATOR_RCURLY))
+	error_at(fallthrough_loc, _("cannot fallthrough final case in switch"));
     }
 
   if (is_default)