Fix JSON encoder accepting NaN and Infinity.

sdk/lib/convert/json.dart

Index: sdk/lib/convert/json.dart
diff --git a/sdk/lib/convert/json.dart b/sdk/lib/convert/json.dart
index efac7acb035435da2f010fbd9cabf975022c04f7..16001e69d0b798a0135691ce468167ce02fae08b 100644
--- a/sdk/lib/convert/json.dart
+++ b/sdk/lib/convert/json.dart
@@ -426,7 +426,7 @@ class _JsonStringifier {
    */
   bool stringifyJsonValue(final object) {
     if (object is num) {
-      // TODO: use writeOn.
+      if (!object.isFinite) return false;
       sink.write(numberToString(object));
       return true;
     } else if (identical(object, true)) {
@@ -449,7 +449,6 @@ class _JsonStringifier {
       sink.write('[');
       if (a.length > 0) {
         stringifyValue(a[0]);
-        // TODO: switch to Iterables.
         for (int i = 1; i < a.length; i++) {
           sink.write(',');
           stringifyValue(a[i]);