Fix test to not assume that JSON.decode returns dynamic.

tests/corelib/string_source_test.dart

Index: tests/corelib/string_source_test.dart
diff --git a/tests/corelib/string_source_test.dart b/tests/corelib/string_source_test.dart
index bf91390b1a0d9d19e057019822bcbbf843a6ee4b..adf05544d341d42e927751db387eb69ac7ca787d 100644
--- a/tests/corelib/string_source_test.dart
+++ b/tests/corelib/string_source_test.dart
@@ -24,7 +24,7 @@ main() {
                        ..writeCharCode(0xdc12)).toString(),
     (new StringBuffer()..writeCharCode(0x10412)).toString(),
     JSON.decode('"\u{10412}"'),
-    JSON.decode('{"\u{10412}":[]}').keys.first
+    (JSON.decode('{"\u{10412}":[]}') as Map).keys.first
   ];
   for (String string in strings) {
     Expect.equals(base.length, string.length);