diff --git a/tools/repl_test.py b/tools/repl_test.py
index 9969e9804c..ce8ef917c8 100644
--- a/tools/repl_test.py
+++ b/tools/repl_test.py
@@ -1,8 +1,8 @@
 # Copyright 2018 the Deno authors. All rights reserved. MIT license.
 import os
-from subprocess import PIPE, Popen
+from subprocess import CalledProcessError, PIPE, Popen
 import sys
-from time import sleep
+import time
 
 from util import build_path, executable_suffix, green_ok
 
@@ -10,20 +10,29 @@ from util import build_path, executable_suffix, green_ok
 class Repl(object):
     def __init__(self, deno_exe):
         self.deno_exe = deno_exe
-        self.warm_up()
+        self._warm_up()
+
+    def _warm_up(self):
+        # This may output an error message about the history file (ignore it).
+        self.input("")
 
     def input(self, *lines, **kwargs):
         exit_ = kwargs.pop("exit", True)
+        sleep_ = kwargs.pop("sleep", 0)
         p = Popen([self.deno_exe], stdout=PIPE, stderr=PIPE, stdin=PIPE)
         try:
+            # Note: The repl takes a >100ms until it's ready.
+            time.sleep(sleep_)
             for line in lines:
                 p.stdin.write(line.encode("utf-8") + b'\n')
+                p.stdin.flush()
+                time.sleep(sleep_)
             if exit_:
                 p.stdin.write(b'deno.exit(0)\n')
             else:
-                sleep(1)  # wait to be killed by js
+                time.sleep(1)  # wait to be killed by js
             out, err = p.communicate()
-        except Exception as e:  # Should this be CalledProcessError?
+        except CalledProcessError as e:
             p.kill()
             p.wait()
             raise
@@ -31,15 +40,14 @@ class Repl(object):
         # Ignore Windows CRLF (\r\n).
         return out.replace('\r\n', '\n'), err.replace('\r\n', '\n'), retcode
 
-    def warm_up(self):
-        # This may output an error message about the history file (ignore it).
-        self.input("")
-
-    def test_function(self):
-        out, err, code = self.input("deno.writeFileSync")
-        assertEqual(out, '[Function: writeFileSync]\n')
-        assertEqual(err, '')
-        assertEqual(code, 0)
+    def run(self):
+        print('repl_test.py')
+        test_names = [name for name in dir(self) if name.startswith("test_")]
+        for t in test_names:
+            self.__getattribute__(t)()
+            sys.stdout.write(".")
+            sys.stdout.flush()
+        print(' {}\n'.format(green_ok()))
 
     def test_console_log(self):
         out, err, code = self.input("console.log('hello')", "'world'")
@@ -47,13 +55,31 @@ class Repl(object):
         assertEqual(err, '')
         assertEqual(code, 0)
 
-    def test_variable(self):
-        out, err, code = self.input("var a = 123;", "a")
-        assertEqual(out, 'undefined\n123\n')
+    def test_exit_command(self):
+        out, err, code = self.input(".exit", "'ignored'", exit=False)
+        assertEqual(out, '')
         assertEqual(err, '')
         assertEqual(code, 0)
 
-    def test_settimeout(self):
+    def test_function(self):
+        out, err, code = self.input("deno.writeFileSync")
+        assertEqual(out, '[Function: writeFileSync]\n')
+        assertEqual(err, '')
+        assertEqual(code, 0)
+
+    def test_multiline(self):
+        out, err, code = self.input("(\n1 + 2\n)")
+        assertEqual(out, '3\n')
+        assertEqual(err, '')
+        assertEqual(code, 0)
+
+    def test_reference_error(self):
+        out, err, code = self.input("not_a_variable")
+        assertEqual(out, '')
+        assertEqual(err, 'ReferenceError: not_a_variable is not defined\n')
+        assertEqual(code, 0)
+
+    def test_set_timeout(self):
         out, err, code = self.input(
             "setTimeout(() => { console.log('b'); deno.exit(0); }, 10)",
             "'a'",
@@ -62,10 +88,13 @@ class Repl(object):
         assertEqual(err, '')
         assertEqual(code, 0)
 
-    def test_reference_error(self):
-        out, err, code = self.input("not_a_variable")
-        assertEqual(out, '')
-        assertEqual(err, 'ReferenceError: not_a_variable is not defined\n')
+    def test_set_timeout_interlaced(self):
+        out, err, code = self.input(
+            "setTimeout(() => console.log('a'), 250)",
+            "setTimeout(() => console.log('b'), 150)",
+            sleep=0.2)
+        assertEqual(out, '1\n2\na\nb\n')
+        assertEqual(err, '')
         assertEqual(code, 0)
 
     def test_syntax_error(self):
@@ -80,40 +109,12 @@ class Repl(object):
         assertEqual(err, 'TypeError: console is not a function\n')
         assertEqual(code, 0)
 
-    def test_multiline(self):
-        out, err, code = self.input("(\n1 + 2\n)")
-        assertEqual(out, '3\n')
+    def test_variable(self):
+        out, err, code = self.input("var a = 123;", "a")
+        assertEqual(out, 'undefined\n123\n')
         assertEqual(err, '')
         assertEqual(code, 0)
 
-    def test_set_timeout(self):
-        # Special treatment
-        p = Popen([self.deno_exe], stdout=PIPE, stderr=PIPE, stdin=PIPE)
-        # Print after 0.1 second
-        p.stdin.write(
-            "setTimeout(() => console.log('HI'), 100)\n".encode("utf-8"))
-        sleep(0.2)  # Wait 0.2 second before proceed
-        out, err = p.communicate()
-        code = p.poll()
-        assertEqual(out.replace('\r\n', '\n'), '1\nHI\n')
-        assertEqual(err.replace('\r\n', '\n'), '')
-        assertEqual(code, 0)
-
-    def test_exit_command(self):
-        out, err, code = self.input(".exit", "'ignored'", exit=False)
-        assertEqual(out, '')
-        assertEqual(err, '')
-        assertEqual(code, 0)
-
-    def run(self):
-        print('repl_test.py')
-        test_names = [name for name in dir(self) if name.startswith("test_")]
-        for t in test_names:
-            self.__getattribute__(t)()
-            sys.stdout.write(".")
-            sys.stdout.flush()
-        print(' {}\n'.format(green_ok()))
-
 
 def assertEqual(left, right):
     if left != right: