Provide and respect HTTP cache headers in pub serve.

lib/src/utils.dart

Index: lib/src/utils.dart
diff --git a/lib/src/utils.dart b/lib/src/utils.dart
index 99b567ae6ff02d4bfa73048d437bf19b108d90b2..a1e848156506c4872331fdba328ff56e7e6e934a 100644
--- a/lib/src/utils.dart
+++ b/lib/src/utils.dart
@@ -521,7 +521,7 @@ Future<Stream> validateStream(Stream stream) {
     // Otherwise, the error came first and the stream is invalid.
     completer.completeError(error, stackTrace);
 
-    // We don't be returning the stream at all in this case, so unsubscribe
+    // We won't be returning the stream at all in this case, so unsubscribe
     // and swallow the error.
     subscription.cancel();
   }, onDone: () {