Add a --reporter flag and expose the expanded reporter.

test/runner/browser/expanded_reporter_test.dart

Index: test/runner/browser/expanded_reporter_test.dart
diff --git a/test/runner/browser/compact_reporter_test.dart b/test/runner/browser/expanded_reporter_test.dart
similarity index 86%
copy from test/runner/browser/compact_reporter_test.dart
copy to test/runner/browser/expanded_reporter_test.dart
index 2f38ff5b80aa5b5dfbe1f2412ec9c661352ba622..a75eecedf5e99c9f9061c4e045fbfbadd1a474c1 100644
--- a/test/runner/browser/compact_reporter_test.dart
+++ b/test/runner/browser/expanded_reporter_test.dart
@@ -35,8 +35,17 @@ void main() {
 }
 """);
 
-    var result = _runTest(
-        ["-p", "content-shell", "-p", "vm", "-j", "1", "test.dart"]);
+    var result = _runTest([
+      "-r",
+      "expanded",
+      "-p",
+      "content-shell",
+      "-p",
+      "vm",
+      "-j",
+      "1",
+      "test.dart"
+    ]);
     expect(result.stdout, contains("[VM]"));
     expect(result.stdout, contains("[Dartium Content Shell]"));
     expect(result.exitCode, equals(0));