Adds a repeatable --url_mapping flag to dartanalyzer

pkg/analyzer/test/options_test.dart

Index: pkg/analyzer/test/options_test.dart
===================================================================
--- pkg/analyzer/test/options_test.dart	(revision 43787)
+++ pkg/analyzer/test/options_test.dart	(working copy)
@@ -30,6 +30,8 @@
       expect(options.sourceFiles, equals(['foo.dart']));
       expect(options.warmPerf, isFalse);
       expect(options.warningsAreFatal, isFalse);
+      expect(options.customUrlMappings, isNotNull);
+      expect(options.customUrlMappings.isEmpty, isTrue);
     });
 
     test('batch', () {
@@ -107,6 +109,17 @@
       expect(options.warningsAreFatal, isTrue);
     });
 
+    test('customUrlMappings', () {
+      CommandLineOptions options = CommandLineOptions.parse([
+          '--dart-sdk', '.',
+          '--url-mapping', 'dart:dummy,/path/to/dummy.dart',
+          'foo.dart']);
+      expect(options.customUrlMappings, isNotNull);
+      expect(options.customUrlMappings.isEmpty, isFalse);
+      expect(options.customUrlMappings['dart:dummy'],
+             equals('/path/to/dummy.dart'));
+    });
+
 //    test('notice unrecognized flags', () {
 //      CommandLineOptions options = new CommandLineOptions.parse(['--bar', '--baz',
 //        'foo.dart']);