Support JSON output for pub build.

sdk/lib/_internal/pub/lib/src/utils.dart

Index: sdk/lib/_internal/pub/lib/src/utils.dart
diff --git a/sdk/lib/_internal/pub/lib/src/utils.dart b/sdk/lib/_internal/pub/lib/src/utils.dart
index e9d4acb0db1c4ecc3369e342d391f19a5619bc77..2584bc42f6f8e1468b15f0e0ad132ce92a28a01d 100644
--- a/sdk/lib/_internal/pub/lib/src/utils.dart
+++ b/sdk/lib/_internal/pub/lib/src/utils.dart
@@ -859,7 +859,20 @@ class ApplicationException implements Exception {
 
 /// A class for command usage exceptions.
 class UsageException extends ApplicationException {
-  UsageException(String message)
+  /// The command usage information.
+  final String usage;
+
+  UsageException(String message, this.usage)
+      : super(message);
+
+  String toString() => "$message\n\n$usage";
+}
+
+/// A class for errors in a command's input data.
+///
+/// This corresponds to the [exit_codes.DATA] exit code.
+class DataException extends ApplicationException {
+  DataException(String message)
       : super(message);
 }