Don't truncate multi-line command descriptions.

lib/command_runner.dart

Index: lib/command_runner.dart
diff --git a/lib/command_runner.dart b/lib/command_runner.dart
index 10b87ce0bfa12e8ba98e03f4f986e267ac7a295a..df1dbf86f1458a0addb9c5a3615dab3422e346e1 100644
--- a/lib/command_runner.dart
+++ b/lib/command_runner.dart
@@ -371,9 +371,15 @@ String _getCommandUsage(Map<String, Command> commands,
   var buffer =
       new StringBuffer('Available ${isSubcommand ? "sub" : ""}commands:');
   for (var name in names) {
+    var lines = commands[name].description.split("\n");
     buffer.writeln();
-    buffer.write('  ${padRight(name, length)}   '
-        '${commands[name].description.split("\n").first}');
+    buffer.write('  ${padRight(name, length)}   ${lines.first}');
+
+    for (var line in lines.skip(1)) {
+      buffer.writeln();
+      buffer.write(' ' * (length + 5));
+      buffer.write(line);
+    }
   }
 
   return buffer.toString();