Support JSON output for pub build.

sdk/lib/_internal/pub/test/build/outputs_error_to_json_test.dart

Index: sdk/lib/_internal/pub/test/build/outputs_error_to_json_test.dart
diff --git a/sdk/lib/_internal/pub/test/build/outputs_error_to_json_test.dart b/sdk/lib/_internal/pub/test/build/outputs_error_to_json_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..074690a26238e598397012138a6fc26885f5e48f
--- /dev/null
+++ b/sdk/lib/_internal/pub/test/build/outputs_error_to_json_test.dart
@@ -0,0 +1,62 @@
+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS d.file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library pub_tests;
+
+import 'package:scheduled_test/scheduled_test.dart';
+
+import '../../lib/src/exit_codes.dart' as exit_codes;
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+
+const TRANSFORMER = """
+import 'dart:async';
+
+import 'package:barback/barback.dart';
+
+class RewriteTransformer extends Transformer {
+  RewriteTransformer.asPlugin();
+
+  String get allowedExtensions => '.txt';
+
+  Future apply(Transform transform) => throw new Exception('oh no!');
+}
+""";
+
+main() {
+  initConfig();
+  integration("outputs error to JSON in a failed build", () {
+    // Loading transformers takes several seconds, so make sure we don't
+    // timeout.
+    currentSchedule.timeout *= 2;
+
+    d.dir(appPath, [
+      d.pubspec({
+        "name": "myapp",
+        "transformers": ["myapp"]
+      }),
+      d.dir("lib", [
+        d.file("transformer.dart", TRANSFORMER)
+      ]),
+      d.dir("web", [
+        d.file("foo.txt", "foo")
+      ])
+    ]).create();
+
+    createLockFile('myapp', pkg: ['barback']);
+
+    schedulePub(args: ["build", "--format", "json"],
+        outputJson: {
+          "buildResult": "failure",
+          "errors": [
+            {
+              "error": startsWith(
+                  "Transform Rewrite on myapp|web/foo.txt threw error: oh no!")
+            }
+          ],
+          "log": []
+        },
+        exitCode: exit_codes.DATA);
+  });
+}