Add firefox jsshell support in testing scripts.

tools/testing/dart/test_runner.dart

Index: tools/testing/dart/test_runner.dart
===================================================================
--- tools/testing/dart/test_runner.dart	(revision 13209)
+++ tools/testing/dart/test_runner.dart	(working copy)
@@ -32,10 +32,13 @@
   /** Command line arguments to the executable. */
   List<String> arguments;
 
+  /** Environment variables set when running the command */
+  Map<String, String> environment;
+
   /** The actual command line that will be executed. */
   String commandLine;
 
-  Command(this.executable, this.arguments) {
+  Command(this.executable, this.arguments, [this.environment = const {}]) {
     if (Platform.operatingSystem == 'windows') {
       // Windows can't handle the first command if it is a .bat file or the like
       // with the slashes going the other direction.
@@ -633,7 +636,8 @@
     }
   }
 
-  VoidFunction makeReadHandler(StringInputStream source, List<String> destination) {
+  VoidFunction makeReadHandler(StringInputStream source,
+                               List<String> destination) {
     void handler () {
       if (source.closed) return;  // TODO(whesse): Remove when bug is fixed.
       var line = source.readLine();
@@ -656,7 +660,11 @@
 
   void runCommand(Command command,
                   void exitHandler(int exitCode)) {
-    process = Process.start(command.executable, command.arguments);
+    ProcessOptions options = new ProcessOptions();
+    options.environment = command.environment;
+    process = Process.start(command.executable,
+                            command.arguments,
+                            options);
     process.onExit = exitHandler;
     process.onError = (e) {
       print("Error starting process:");