Add dependency checker special case for out/build_date.txt.

Test: Future TH presubmit.
Change-Id: I4f80cf12be4aa1a3040b38ac7c6bed593aefd65e

ui/build/test_build.go

@@ -68,6 +68,7 @@ func testForDanglingRules(ctx Context, config Config) {
 	miniBootstrapDir := filepath.Join(outDir, "soong", ".minibootstrap")
 	modulePathsDir := filepath.Join(outDir, ".module_paths")
 	variablesFilePath := filepath.Join(outDir, "soong", "soong.variables")
+
 	// dexpreopt.config is an input to the soong_docs action, which runs the
 	// soong_build primary builder. However, this file is created from $(shell)
 	// invocation at Kati parse time, so it's not an explicit output of any
@@ -75,6 +76,9 @@ func testForDanglingRules(ctx Context, config Config) {
 	// treated as an source file.
 	dexpreoptConfigFilePath := filepath.Join(outDir, "soong", "dexpreopt.config")
 
+	// out/build_date.txt is considered a "source file"
+	buildDatetimeFilePath := filepath.Join(outDir, "build_date.txt")
+
 	danglingRules := make(map[string]bool)
 
 	scanner := bufio.NewScanner(stdout)
@@ -88,7 +92,8 @@ func testForDanglingRules(ctx Context, config Config) {
 			strings.HasPrefix(line, miniBootstrapDir) ||
 			strings.HasPrefix(line, modulePathsDir) ||
 			line == variablesFilePath ||
-			line == dexpreoptConfigFilePath {
+			line == dexpreoptConfigFilePath ||
+			line == buildDatetimeFilePath {
 			// Leaf node is in one of Soong's bootstrap directories, which do not have
 			// full build rules in the primary build.ninja file.
 			continue