Avoid "cannot create a Trace from null" errors in pub.

sdk/lib/_internal/pub/lib/src/utils.dart

Index: sdk/lib/_internal/pub/lib/src/utils.dart
diff --git a/sdk/lib/_internal/pub/lib/src/utils.dart b/sdk/lib/_internal/pub/lib/src/utils.dart
index 6d777135d9abc2b69d80448f8b652826872db08b..4a63ca3beaaeda001e13433919e889818d6b4d89 100644
--- a/sdk/lib/_internal/pub/lib/src/utils.dart
+++ b/sdk/lib/_internal/pub/lib/src/utils.dart
@@ -144,7 +144,14 @@ Future captureErrors(Future callback(), {bool captureStackChains: false}) {
   var wrappedCallback = () {
     new Future.sync(callback).then(completer.complete)
         .catchError((e, stackTrace) {
-      completer.completeError(e, new Chain.forTrace(stackTrace));
+      // [stackTrace] can be null if we're running without [captureStackChains],
+      // since dart:io will often throw errors without stack traces.
+      if (stackTrace != null) {
+        stackTrace = new Chain.forTrace(stackTrace);
+      } else {
+        stackTrace = new Chain([]);
+      }
+      completer.completeError(e, stackTrace);
     });
   };
 
@@ -152,7 +159,12 @@ Future captureErrors(Future callback(), {bool captureStackChains: false}) {
     Chain.capture(wrappedCallback, onError: completer.completeError);
   } else {
     runZoned(wrappedCallback, onError: (e, stackTrace) {
-      completer.completeError(e, new Chain([new Trace.from(stackTrace)]));
+      if (stackTrace == null) {
+        stackTrace = new Chain([new Trace.from(stackTrace)]);
+      } else {
+        stackTrace = new Chain([]);
+      }
+      completer.completeError(e, stackTrace);
     });
   }