pkg/args: expose immutable collections

pkg/args/lib/src/usage.dart

Index: pkg/args/lib/src/usage.dart
diff --git a/pkg/args/lib/src/usage.dart b/pkg/args/lib/src/usage.dart
index 45893a5e7c3030ead5602dfc9228f3615915f503..c3dd68eaab59d3b33170e04280ec270f49c9c0d4 100644
--- a/pkg/args/lib/src/usage.dart
+++ b/pkg/args/lib/src/usage.dart
@@ -144,13 +144,13 @@ class Usage {
     columnWidths = [abbr, title];
   }
 
-  newline() {
+  void newline() {
     newlinesNeeded++;
     currentColumn = 0;
     numHelpLines = 0;
   }
 
-  write(int column, String text) {
+  void write(int column, String text) {
     var lines = text.split('\n');
 
     // Strip leading and trailing empty lines.
@@ -167,7 +167,7 @@ class Usage {
     }
   }
 
-  writeLine(int column, String text) {
+  void writeLine(int column, String text) {
     // Write any pending newlines.
     while (newlinesNeeded > 0) {
       buffer.write('\n');
@@ -208,7 +208,7 @@ class Usage {
     }
   }
 
-  buildAllowedList(Option option) {
+  String buildAllowedList(Option option) {
     var allowedBuffer = new StringBuffer();
     allowedBuffer.write('[');
     bool first = true;