gn: test and fix for swapped -= operator

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Review URL: https://chromiumcodereview.appspot.com/23484029

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@222619 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/tools/gn/parser_unittest.cc b/tools/gn/parser_unittest.cc
index 06464a2..7ef5ff6 100644
--- a/tools/gn/parser_unittest.cc
+++ b/tools/gn/parser_unittest.cc
@@ -475,3 +475,7 @@
 TEST(Parser, HangingIf) {
   DoParserErrorTest("if", 1, 1);
 }
+
+TEST(Parser, NegatingList) {
+  DoParserErrorTest("executable(\"wee\") { sources =- [ \"foo.cc\" ] }", 1, 30);
+}