Don't quit debugger when the program terminates, only after `quit`.

tests/debugger/unused_optional_expected.txt

 Starting session. Type 'help' for a list of commands.
 
 > b main
 ### set breakpoint id: '0' method: 'main' bytecode index: '0'
 > r
 tests/debugger/unused_optional_test.dart:12:17
 12     Expect.equals(1, 1);
 > s
 tests/debugger/unused_optional_test.dart:12:20
 12     Expect.equals(1, 1);
 > s
 third_party/dart/pkg/expect/lib/expect.dart:93:65
 93     static void equals(var expected, var actual, [String reason = null]) {
 > l
 third_party/dart/pkg/expect/lib/expect.dart:93:65
 88     }
 89   
 90     /**
 91      * Checks whether the expected and actual values are equal (using `==`).
 92      */
 93     static void equals(var expected, var actual, [String reason = null]) {
 94       if (expected == actual) return;
 95       String msg = _getMessage(reason);
 96       if (expected is String && actual is String) {
 97         String stringDifference = _stringDifference(expected, actual);
 98         if (stringDifference != null) {
 > c
 ### process terminated
+> q