Factor out some common logic from the launchers.

lib/src/runner/browser/internet_explorer.dart

Index: lib/src/runner/browser/internet_explorer.dart
diff --git a/lib/src/runner/browser/internet_explorer.dart b/lib/src/runner/browser/internet_explorer.dart
index 920b375cb18f8f75141cc957d862afec0800aeb7..0d034b15fa4b7a567aa90dc7f42c15f38de38e57 100644
--- a/lib/src/runner/browser/internet_explorer.dart
+++ b/lib/src/runner/browser/internet_explorer.dart
@@ -5,81 +5,33 @@
 library test.runner.browser.internet_explorer;
 
 import 'dart:async';
-import 'dart:convert';
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
-import 'package:stack_trace/stack_trace.dart';
 
-import '../../utils.dart';
-import '../application_exception.dart';
 import 'browser.dart';
 
 /// A class for running an instance of Internet Explorer.
 ///
 /// Any errors starting or running the process are reported through [onExit].
-class InternetExplorer implements Browser {
-  /// The underlying process.
-  Process _process;
+class InternetExplorer extends Browser {
+  final name = "Internet Explorer";
 
-  Future get onExit => _onExitCompleter.future;
-  final _onExitCompleter = new Completer();
-
-  /// A future that completes when the browser process has started.
-  ///
-  /// This is used to ensure that [close] works regardless of when it's called.
-  Future get _onProcessStarted => _onProcessStartedCompleter.future;
-  final _onProcessStartedCompleter = new Completer();
+  InternetExplorer(url, {String executable})
+      : super(() => _startBrowser(url, executable));
 
   /// Starts a new instance of Internet Explorer open to the given [url], which
   /// may be a [Uri] or a [String].
   ///
   /// If [executable] is passed, it's used as the Internet Explorer executable.
-  /// Otherwise the default executable name will be used.
-  InternetExplorer(url, {String executable}) {
+  /// Otherwise the default executable name for the current OS will be used.
+  static Future<Process> _startBrowser(url, [String executable]) {
     if (executable == null) executable = _defaultExecutable();
-
-    // Don't return a Future here because there's no need for the caller to wait
-    // for the process to actually start. They should just wait for the HTTP
-    // request instead.
-    invoke(() async {
-      try {
-        var process = await Process.start(
-            executable, ['-extoff', url.toString()]);
-
-        _process = process;
-        _onProcessStartedCompleter.complete();
-
-        // TODO(nweiz): the browser's standard output is almost always useless
-        // noise, but we should allow the user to opt in to seeing it.
-        var exitCode = await _process.exitCode;
-        if (exitCode != 0) {
-          var error = await UTF8.decodeStream(_process.stderr);
-          throw new ApplicationException(
-              "Internet Explorer failed with exit code $exitCode:\n$error");
-        }
-
-        _onExitCompleter.complete();
-      } catch (error, stackTrace) {
-        if (stackTrace == null) stackTrace = new Trace.current();
-        _onExitCompleter.completeError(
-            new ApplicationException(
-                "Failed to start Internet Explorer: "
-                    "${getErrorMessage(error)}."),
-            stackTrace);
-      }
-    });
-  }
-
-  Future close() {
-    _onProcessStarted.then((_) => _process.kill());
-
-    // Swallow exceptions. The user should explicitly use [onExit] for these.
-    return onExit.catchError((_) {});
+    return Process.start(executable, ['-extoff', url.toString()]);
   }
 
   /// Return the default executable for the current operating system.
-  String _defaultExecutable() {
+  static String _defaultExecutable() {
     // Internet Explorer could be installed in several places on Windows. The
     // only way to find it is to check.
     var prefixes = [