Ensure that pub doesn't double-complete its captureErrors future.

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 4daec903e75bca57081a61caad8008ec9e37dba0..09575d7bf8e9e54d733ade12dfd87f3ab9adfc47 100644
--- a/sdk/lib/_internal/pub/lib/src/utils.dart
+++ b/sdk/lib/_internal/pub/lib/src/utils.dart
@@ -104,12 +104,14 @@ Future captureErrors(Future callback(), {bool captureStackChains: false}) {
       } else {
         stackTrace = new Chain([]);
       }
-      completer.completeError(e, stackTrace);
+      if (!completer.isCompleted) completer.completeError(e, stackTrace);
     });
   };
 
   if (captureStackChains) {
-    Chain.capture(wrappedCallback, onError: completer.completeError);
+    Chain.capture(wrappedCallback, onError: (error, stackTrace) {
+      if (!completer.isCompleted) completer.completeError(error, stackTrace);
+    });
   } else {
     runZoned(wrappedCallback, onError: (e, stackTrace) {
       if (stackTrace == null) {
@@ -117,7 +119,7 @@ Future captureErrors(Future callback(), {bool captureStackChains: false}) {
       } else {
         stackTrace = new Chain([new Trace.from(stackTrace)]);
       }
-      completer.completeError(e, stackTrace);
+      if (!completer.isCompleted) completer.completeError(e, stackTrace);
     });
   }