Change IllegalArgumentException to ArgumentError.

pkg/args/lib/args.dart

Index: pkg/args/lib/args.dart
diff --git a/pkg/args/lib/args.dart b/pkg/args/lib/args.dart
index 7c51a08773c72e2a40b623343755b3020e99a321..52e7350d7f73771d3bec36e8c760ddda944ece19 100644
--- a/pkg/args/lib/args.dart
+++ b/pkg/args/lib/args.dart
@@ -220,7 +220,7 @@ class ArgParser {
       _optionNames = <String>[];
 
   /**
-   * Defines a flag. Throws an [IllegalArgumentException] if:
+   * Defines a flag. Throws an [ArgumentError] if:
    *
    * * There is already an option named [name].
    * * There is already an option using abbreviation [abbr].
@@ -232,7 +232,7 @@ class ArgParser {
   }
 
   /**
-   * Defines a value-taking option. Throws an [IllegalArgumentException] if:
+   * Defines a value-taking option. Throws an [ArgumentError] if:
    *
    * * There is already an option with name [name].
    * * There is already an option using abbreviation [abbr].
@@ -250,19 +250,19 @@ class ArgParser {
       bool allowMultiple = false]) {
     // Make sure the name isn't in use.
     if (_options.containsKey(name)) {
-      throw new IllegalArgumentException('Duplicate option "$name".');
+      throw new ArgumentError('Duplicate option "$name".');
     }
 
     // Make sure the abbreviation isn't too long or in use.
     if (abbr != null) {
       if (abbr.length > 1) {
-        throw new IllegalArgumentException(
+        throw new ArgumentError(
             'Abbreviation "$abbr" is longer than one character.');
       }
 
       var existing = _findByAbbr(abbr);
       if (existing != null) {
-        throw new IllegalArgumentException(
+        throw new ArgumentError(
             'Abbreviation "$abbr" is already used by "${existing.name}".');
       }
     }
@@ -504,7 +504,7 @@ class ArgParser {
    */
   getDefault(String option) {
     if (!_options.containsKey(option)) {
-      throw new IllegalArgumentException('No option named $option');
+      throw new ArgumentError('No option named $option');
     }
     return _options[option].defaultValue;
   }
@@ -531,7 +531,7 @@ class ArgResults {
   /** Gets the parsed command-line option named [name]. */
   operator [](String name) {
     if (!_options.containsKey(name)) {
-      throw new IllegalArgumentException(
+      throw new ArgumentError(
           'Could not find an option named "$name".');
     }