Fix various semantic nits that Fasta will detect.

sdk/lib/_internal/js_runtime/lib/js_helper.dart

Index: sdk/lib/_internal/js_runtime/lib/js_helper.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/js_helper.dart b/sdk/lib/_internal/js_runtime/lib/js_helper.dart
index f44f922f49644d4db51a5b253ee0f4c1d98e8024..8174b0bd2812eb8e3ca6224ea22587227dc4ffba 100644
--- a/sdk/lib/_internal/js_runtime/lib/js_helper.dart
+++ b/sdk/lib/_internal/js_runtime/lib/js_helper.dart
@@ -2350,7 +2350,7 @@ unwrapException(ex) {
     // argument to a function that does not allow a range that includes that
     // number. Translate to a Dart ArgumentError with the same message.
     // TODO(sra): Translate to RangeError.
-    String message = tryStringifyException(ex);
+    message = tryStringifyException(ex);

[ahe, 2017/05/23 14:44:27]

Already defined and used above in this scope.

     if (message is String) {
       message = JS('String', r'#.replace(/^RangeError:\s*/, "")', message);
     }