Support JSON output for pub build.

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

Index: sdk/lib/_internal/pub/test/build/missing_web_directory_test.dart
diff --git a/sdk/lib/_internal/pub/test/build/missing_web_directory_test.dart b/sdk/lib/_internal/pub/test/build/missing_web_directory_test.dart
index ccf4264d5267c81ad0f34fbbf5eb82f23fd63579..953ee7427d8212208d2a3518d1b5656f8d7d8e52 100644
--- a/sdk/lib/_internal/pub/test/build/missing_web_directory_test.dart
+++ b/sdk/lib/_internal/pub/test/build/missing_web_directory_test.dart
@@ -16,4 +16,15 @@ main() {
         error: 'Directory "web" does not exist.',
         exitCode: exit_codes.DATA);
   });
+
+  integration("fails if 'web' doesn't exist and no directory is specified "
+      "with JSON output", () {
+    d.appDir().create();
+
+    schedulePub(args: ["build", "--format", "json"],
+        outputJson: {
+          "error": 'Directory "web" does not exist.'
+        },
+        exitCode: exit_codes.DATA);
+  });
 }