Allow command-line API to pass in selection to preserve. Fix #194.

test/command_line_test.dart

Index: test/command_line_test.dart
diff --git a/test/command_line_test.dart b/test/command_line_test.dart
index b2cf84a0d7f5114efe593265714bb8bf7d1eb705..4d1e0d9a3ab9861ca955188752e03380bcbf949d 100644
--- a/test/command_line_test.dart
+++ b/test/command_line_test.dart
@@ -147,6 +147,41 @@ void main() {
     });
   });
 
+  group("--preserve", () {
+    test("errors if given paths.", () {
+      var process = runFormatter(["--preserve", "path", "another"]);
+      process.shouldExit(64);
+    });
+
+    test("errors on wrong number of components.", () {
+      var process = runFormatter(["--preserve", "1"]);
+      process.shouldExit(64);
+
+      process = runFormatter(["--preserve", "1:2:3"]);
+      process.shouldExit(64);
+    });
+
+    test("errors on non-integer component.", () {
+      var process = runFormatter(["--preserve", "1:2.3"]);
+      process.shouldExit(64);
+    });
+
+    test("updates selection.", () {
+      var process = runFormatter(["--preserve", "6:10", "-m"]);
+      process.writeLine(unformattedSource);
+      process.closeStdin();
+
+      var json = JSON.encode({
+        "path": "<stdin>",
+        "source": formattedSource,
+        "selection": {"offset": 5, "length": 9}
+      });
+
+      process.stdout.expect(json);
+      process.shouldExit();
+    });
+  });
+
   group("with no paths", () {
     test("errors on --overwrite.", () {
       var process = runFormatter(["--overwrite"]);