Fix three missing return warnings.

Change-Id: I69c5684a40bf46ac6a2b1ed304ea593b9d073922
Reviewed-on: https://dart-review.googlesource.com/68381
Commit-Queue: Devon Carew <devoncarew@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

pkg/analyzer_plugin/test/integration/support/integration_tests.dart

@@ -951,7 +951,5 @@ abstract class _RecursiveMatcher extends Matcher {
    * Create a [MismatchDescriber] describing a mismatch with a simple string.
    */
   MismatchDescriber simpleDescription(String description) =>
-      (Description mismatchDescription) {
-        mismatchDescription.add(description);
-      };
+      (Description mismatchDescription) => mismatchDescription.add(description);
 }

tools/testing/dart/browser_controller.dart

@@ -1362,13 +1362,12 @@ class BrowserTestingServer {
       } else {
         textResponse = new Future<String>.value("");
       }
-      request.response.done.catchError((error) {
+      request.response.done.catchError((error) async {
         if (!underTermination) {
-          return textResponse.then((String text) {
-            print("URI ${request.uri}");
-            print("textResponse $textResponse");
-            throw "Error returning content to browser: $error";
-          });
+          String text = await textResponse;
+          print("URI ${request.uri}");
+          print("text $text");
+          throw "Error returning content to browser: $error";
         }
       });
       textResponse.then((String text) async {

tools/testing/dart/command.dart

@@ -656,7 +656,7 @@ class MakeSymlinkCommand extends ScriptCommand {
       var link = new io.Link(_link);
 
       return link.exists().then((bool exists) {
-        if (exists) return link.delete();
+        if (exists) link.deleteSync();
       }).then((_) => link.create(_target));
     }).then((_) {
       return new ScriptCommandOutput(this, Expectation.pass, "", watch.elapsed);