Add "environment" parameter to spawnUri for String.fromEnvironment support.

runtime/lib/isolate_patch.dart

Index: runtime/lib/isolate_patch.dart
diff --git a/runtime/lib/isolate_patch.dart b/runtime/lib/isolate_patch.dart
index 51b7ac0123147c1e9103d2875bf0efae471fcf42..ca374ff7d641ba529da3df9eb4076f8cdc40864b 100644
--- a/runtime/lib/isolate_patch.dart
+++ b/runtime/lib/isolate_patch.dart
@@ -310,12 +310,14 @@ patch class Isolate {
       Uri uri, List<String> args, var message,
       {bool paused: false,
        bool checked,
+       Map<String, String> environment,
        Uri packageRoot,
        Map<String, Uri> packages,
        bool errorsAreFatal,
        SendPort onExit,
        SendPort onError}) {
     RawReceivePort readyPort;
+    if (environment != null) throw new UnimplementedError("environment");
     try {
       // The VM will invoke [_startIsolate] and not `main`.
       // TODO: Handle [packages].