Fix a Dart 2 runtime cast failure

We get a List<dynamic> from jsonDecode which cannot be cast to a
List<String>.

Change-Id: Ifd506a2ecdac2996dfe52d4f2e98c83f2cbbf421
Reviewed-on: https://dart-review.googlesource.com/60023
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Kevin Millikin <kmillikin@google.com>
diff --git a/pkg/front_end/tool/_fasta/entry_points.dart b/pkg/front_end/tool/_fasta/entry_points.dart
index b2a1d8f..3af6703 100644
--- a/pkg/front_end/tool/_fasta/entry_points.dart
+++ b/pkg/front_end/tool/_fasta/entry_points.dart
@@ -100,7 +100,8 @@
   run() async {
     await for (String line in lines) {
       try {
-        if (await batchCompileArguments(jsonDecode(line))) {
+        if (await batchCompileArguments(
+            new List<String>.from(jsonDecode(line)))) {
           stdout.writeln(">>> TEST OK");
         } else {
           stdout.writeln(">>> TEST FAIL");