Add a JSON reporter.

test/runner/compact_reporter_test.dart

Index: test/runner/compact_reporter_test.dart
diff --git a/test/runner/compact_reporter_test.dart b/test/runner/compact_reporter_test.dart
index 5380d85e9f55ad2a7677c1d88af133ab67c4ad61..47b9db9112471c41cda8ad054539ff05e90de1c0 100644
--- a/test/runner/compact_reporter_test.dart
+++ b/test/runner/compact_reporter_test.dart
@@ -16,7 +16,7 @@ void main() {
   test("reports when no tests are run", () {
     d.file("test.dart", "void main() {}").create();
 
-    var test = runTest(["test.dart"], compact: true);
+    var test = runTest(["test.dart"], reporter: "compact");
     test.stdout.expect(consumeThrough(contains("No tests ran.")));
     test.shouldExit(0);
   });
@@ -76,7 +76,7 @@ void main() {
 }
 """).create();
 
-    var test = runTest(["--verbose-trace", "test.dart"], compact: true);
+    var test = runTest(["--verbose-trace", "test.dart"], reporter: "compact");
     test.stdout.expect(consumeThrough(contains("dart:isolate-patch")));
     test.shouldExit(1);
   });
@@ -406,7 +406,7 @@ $tests
 
   d.file("test.dart", dart).create();
 
-  var test = runTest(["test.dart"], compact: true);
+  var test = runTest(["test.dart"], reporter: "compact");
   test.shouldExit();
 
   schedule(() async {