Add a 'secure' constructor to Random in dart:math returning a cryptographically

sdk/lib/_internal/js_runtime/lib/math_patch.dart

Index: sdk/lib/_internal/js_runtime/lib/math_patch.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/math_patch.dart b/sdk/lib/_internal/js_runtime/lib/math_patch.dart
index 34476f24d8a78c21d3e4f119810c6c1226925a96..1cf3be29548c041c1748eef65970b430884804d5 100644
--- a/sdk/lib/_internal/js_runtime/lib/math_patch.dart
+++ b/sdk/lib/_internal/js_runtime/lib/math_patch.dart
@@ -60,6 +60,12 @@ class Random {
   @patch
   factory Random([int seed]) =>
       (seed == null) ? const _JSRandom() : new _Random(seed);
+
+  @patch
+  factory Random.secure() {
+    throw new UnsupportedError(
+        "No source of cryptographically secure random numbers available.");
+  }
 }
 
 class _JSRandom implements Random {