Give all async exceptions in pub some sort of stack trace.

utils/pub/curl_client.dart

Index: utils/pub/curl_client.dart
diff --git a/utils/pub/curl_client.dart b/utils/pub/curl_client.dart
index 20a1027d48bf0eb431b476dbc8f4d2ab0d44ef53..48bc756b0fa330482d32a9c16434b1737070f7b7 100644
--- a/utils/pub/curl_client.dart
+++ b/utils/pub/curl_client.dart
@@ -135,6 +135,14 @@ class CurlClient extends http.BaseClient {
       ]);
     }
 
+    // TODO(nweiz): remove this when issue 4061 is fixed.
+    var stackTrace;
+    try {
+      throw null;

[Bob Nystrom, 2012/11/29 22:25:40]

Throwing null is forbidden and causes a different kind of exception to be
thrown. It doesn't really matter much, but I think it would be slightly less
weird to throw a non-null value. null is the one value in all of Dart that has
special throw semantics. Maybe throw 'dummy';?

[nweiz, 2012/11/29 22:39:02]

Done.

+    } catch (_, localStackTrace) {
+      stackTrace = localStackTrace;
+    }
+
     var completer = new Completer();
     resetCallbacks() {
       process.stdout.onData = null;
@@ -151,7 +159,7 @@ class CurlClient extends http.BaseClient {
     };
     process.stdout.onError = (e) {
       resetCallbacks();
-      completer.completeException(e);
+      completer.completeException(e, stackTrace);
     };
     process.stdout.onClosed = () {
       resetCallbacks();