Don't close "pub serve" before we've read its output in tests.

sdk/lib/_internal/pub/test/serve/warns_on_assets_paths_test.dart

Index: sdk/lib/_internal/pub/test/serve/warns_on_assets_paths_test.dart
diff --git a/sdk/lib/_internal/pub/test/serve/warns_on_assets_paths_test.dart b/sdk/lib/_internal/pub/test/serve/warns_on_assets_paths_test.dart
index 2f0cfe63a59812d1f644772a59f7a13a204aae28..05f99335d8e4e6b89f279cee3c7720d580b5d75c 100644
--- a/sdk/lib/_internal/pub/test/serve/warns_on_assets_paths_test.dart
+++ b/sdk/lib/_internal/pub/test/serve/warns_on_assets_paths_test.dart
@@ -35,10 +35,10 @@ main() {
 
     var pub = pubServe();
     waitForBuildSuccess();
-    endPubServe();
 
     var assetsPath = path.join('web', 'assets');
     pub.stderr.expect(consumeThrough(matches(getWarningRegExp(assetsPath))));
+    endPubServe();
   });
 
   integration('warns user about assets dir nested anywhere in "web"', () {
@@ -54,10 +54,10 @@ main() {
 
     var pub = pubServe();
     waitForBuildSuccess();
-    endPubServe();
 
     var assetsPath = path.join('web', 'foo', 'assets');
     pub.stderr.expect(consumeThrough(matches(getWarningRegExp(assetsPath))));
+    endPubServe();
   });
 
   integration('warns user about assets file in the root of "web"', () {
@@ -71,10 +71,10 @@ main() {
 
     var pub = pubServe();
     waitForBuildSuccess();
-    endPubServe();
 
     var assetsPath = path.join('web', 'assets');
     pub.stderr.expect(consumeThrough(matches(getWarningRegExp(assetsPath))));
+    endPubServe();
   });
 
   integration('warns user about assets file nested anywhere in "web"', () {
@@ -90,10 +90,10 @@ main() {
 
     var pub = pubServe();
     waitForBuildSuccess();
-    endPubServe();
 
     var assetsPath = path.join('web', 'foo', 'assets');
     pub.stderr.expect(consumeThrough(matches(getWarningRegExp(assetsPath))));
+    endPubServe();
   });
 
   integration('does not warn if no assets dir or file anywhere in "web"', () {