Avoid sending needless data between isolates in "pub serve" and "pub build"

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 8a63d52579beec62bf94877919ada0c0be509af6..c17546e7be8489e8c2794764fcf2b92acee9ec17 100644
--- a/sdk/lib/_internal/pub/lib/src/utils.dart
+++ b/sdk/lib/_internal/pub/lib/src/utils.dart
@@ -393,6 +393,25 @@ Stream mergeStreams(Stream stream1, Stream stream2) {
   return controller.stream;
 }
 
+/// Returns a [Stream] that will emit the same values as the stream returned by
+/// [callback].
+///
+/// [callback] will only be called when the returned [Stream] gets a subscriber.
+Stream callbackStream(Stream callback()) {
+  var subscription;
+  var controller;
+  controller = new StreamController(onListen: () {
+    subscription = callback().listen(controller.add,
+        onError: controller.addError,
+        onDone: controller.close);
+  },
+      onCancel: () => subscription.cancel(),
+      onPause: () => subscription.pause(),
+      onResume: () => subscription.resume(),
+      sync: true);
+  return controller.stream;
+}
+
 /// A regular expression matching a trailing CR character.
 final _trailingCR = new RegExp(r"\r$");