Make catch variables final.

pkg/front_end/test/fasta/expressions.dart.direct.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 static method foo({dynamic fisk = null}) → dynamic {
   core::print(fisk);
 }
 static method caller(dynamic f) → dynamic {
   f.call();
 }
 static method main() → dynamic {
   core::int i = 0;
   core::print(i.==(1) ? "bad" : "good");
   core::print("${i}");
   core::print("'${i}'");
   core::print(" '${i}' ");
   core::print(" '${i}' '${i}'");
   core::print(" '${i}' '${i}'");
   core::print("foobar");
   core::print(" '${i}' '${i}' '${i}' '${i}'");
   try {
     throw "fisk";
   }
-  on core::String catch(dynamic e, dynamic s) {
+  on core::String catch(final dynamic e, final dynamic s) {
     core::print(e);
     if(!s.==(null))
       core::print(s);
   }
   for (; false; ) {
   }
   dynamic list = <dynamic>["Hello, World!"];
   core::print(list.[](i));
   list.[]=(i, "Hello, Brave New World!");
   core::print(list.[](i));
@@ skipping 30 matching lines @@
   self::caller(({dynamic x}) → dynamic {
     core::print("<anon> was called with ${x}");
   });
   core::print(core::int.toString());
   core::print(core::int);
   core::print(let final dynamic #t5 = core::int in let final dynamic #t6 = #t5.toString() in #t5);
   try {
     core::print(throw new core::NoSuchMethodError::•(null, #toString, <dynamic>[], <dynamic, dynamic>{}, null));
     throw "Shouldn't work";
   }
-  on core::NoSuchMethodError catch(dynamic e) {
+  on core::NoSuchMethodError catch(final dynamic e) {
     core::print("As expected: ${e}");
   }
   core::print(core::int::parse("42"));
 }