Fix edge cases where a FormatException source argument starts with a newline.

tests/corelib/format_exception_test.dart

Index: tests/corelib/format_exception_test.dart
diff --git a/tests/corelib/format_exception_test.dart b/tests/corelib/format_exception_test.dart
index 121494cadbf8edce916ece58ea7577df4c26d0be..0222b495b511ad4c1ecc7121e0206116181bf3aa 100644
--- a/tests/corelib/format_exception_test.dart
+++ b/tests/corelib/format_exception_test.dart
@@ -70,4 +70,18 @@ main() {
        "FormatException: message (at line 10, character ${half + 1})\n"
        "...${longline.substring(half - 36, half + 36)}...\n"
        "${' ' * 39}^\n");
+
+  var sourceNL = "\nsource with leading NL";
+  e = new FormatException("message", sourceNL, 2);
+  test(e, "message", sourceNL, 2,
+       "FormatException: message (at line 2, character 2)\n"
+       "source with leading NL\n"
+       " ^\n");
+
+  var sourceNL2 = "\n\nsource with leading NL";
+  e = new FormatException("message", sourceNL2, 2);
+  test(e, "message", sourceNL2, 2,
+       "FormatException: message (at line 3, character 1)\n"
+       "source with leading NL\n"
+       "^\n");
 }