Properly map function names with named arguments.

test/source_map_stack_trace_test.dart

Index: test/source_map_stack_trace_test.dart
diff --git a/test/source_map_stack_trace_test.dart b/test/source_map_stack_trace_test.dart
index 411bf291cfe1ab125554cb1cd1f9bca27ae904e2..12ae7b189fe39a440dd8e6a50c6def5b1482ea2c 100644
--- a/test/source_map_stack_trace_test.dart
+++ b/test/source_map_stack_trace_test.dart
@@ -153,6 +153,11 @@ foo.dart.js 10:11  baz
       expect(_prettify(r"foo$1234"), equals("foo"));
     });
 
+    test("named arguments", () {
+      expect(_prettify(r"foo$1$bar"), equals("foo"));
+      expect(_prettify(r"foo$123$bar$bang$qux"), equals("foo"));
+    });
+
     test("closures", () {
       expect(_prettify("foo_closure.call"), equals("foo.<fn>"));
     });