Reapply "Rename mappedBy to map."

This reverts commit r17907.

TBR.

Review URL: https://codereview.chromium.org//12090093

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/path@17918 260f80e4-7a28-3924-810f-c04153c831b5
diff --git a/lib/path.dart b/lib/path.dart
index 854d4da..3bd2332 100644
--- a/lib/path.dart
+++ b/lib/path.dart
@@ -179,7 +179,7 @@
     var message = new StringBuffer();
     message.add("$method(");
     message.add(args.take(numArgs)
-        .mappedBy((arg) => arg == null ? "null" : '"$arg"')
+        .map((arg) => arg == null ? "null" : '"$arg"')
         .join(", "));
     message.add("): part ${i - 1} was null, but part $i was not.");
     throw new ArgumentError(message.toString());