Add a "pub global run" command.

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 b3d585db92a97b0ebc12d379dd741e25fa0fe382..68c0c74188ba3c9add56553c9b6146d2dcf1dad0 100644
--- a/sdk/lib/_internal/pub/lib/src/utils.dart
+++ b/sdk/lib/_internal/pub/lib/src/utils.dart
@@ -845,6 +845,7 @@ void fail(String message, [innerError, StackTrace innerTrace]) {
 /// failed because of invalid input data.
 ///
 /// This will report the error and cause pub to exit with [exit_codes.DATA].
-void dataError(String message) {
-  throw new DataException(message);
-}
+void dataError(String message) => throw new DataException(message);
+
+/// Throw a [UsageException] for a usage error of a command with [message].
+void usageError(String message) => throw new UsageException(message);