Fix a bug in `gn analyze` related to build-only changes.

tools/gn/analyzer_unittest.cc

Index: tools/gn/analyzer_unittest.cc
diff --git a/tools/gn/analyzer_unittest.cc b/tools/gn/analyzer_unittest.cc
index dba0e7701dd8a7d8af7feb3e2a3ad02acd8909d7..4d5c8d461dea7cc6a36a0546f769d196f30a74bd 100644
--- a/tools/gn/analyzer_unittest.cc
+++ b/tools/gn/analyzer_unittest.cc
@@ -200,3 +200,20 @@ TEST_F(AnalyzerTest, WrongInputFields) {
       "\"invalid_targets\":[]"
       "}");
 }
+
+TEST_F(AnalyzerTest, BuildFilesWereModified) {
+  // This tests that if a build file is modified, we bail out early with
+  // "Found dependency (all)" error since we can't handle changes to
+  // build files yet (crbug.com/555273).
+  RunBasicTest(
+      "{"
+      "  \"files\": [ \"//a.cc\", \"//BUILD.gn\" ],"
+      "  \"additional_compile_targets\": [],"
+      "  \"test_targets\": [ \"//:a\" ]"
+      "}",
+      "{"
+      "\"compile_targets\":[],"
+      "\"status\":\"Found dependency (all)\","
+      "\"test_targets\":[\"//:a\"]"
+      "}");
+}