[dart2wasm] Use JS Math.random to initialize non-secure Random seed

When a seed is not provided to `Random`, call JS `Math.random` to get
two doubles between 0.0 and 1.0, scale the doubles to 32-bit integers,
and combine to a single 64-bit.

Similar to VM, this seed is generated once during the lifetime of the
program and then updated using the PRNG class `_Random`.

Fixes #56609.

Change-Id: Iaf846c63ff26d353ba6c4a94f61b0bab32624509
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/383241
Reviewed-by: Martin Kustermann <[email protected]>
Commit-Queue: Ömer Ağacan <[email protected]>

sdk/lib/_internal/wasm/lib/math_patch.dart

@@ -226,8 +226,11 @@ class _Random implements Random {
 
   static int _setupSeed(int seed) => mix64(seed);
 
-  // TODO: Make this actually random
-  static int _initialSeed() => 0xCAFEBABEDEADBEEF;
+  static int _initialSeed() {
+    final low = (_jsMath.random() * 4294967295.0).toInt();
+    final high = (_jsMath.random() * 4294967295.0).toInt();
+    return ((high << 32) | low);
+  }
 
   static int _nextSeed() {
     // Trigger the PRNG once to change the internal state.
@@ -248,6 +251,18 @@ extension _JSCryptoGetRandomValues on _JSCrypto {
   external void getRandomValues(JSUint8Array array);
 }
 
+@JS('Math')
+external _JSMath get _jsMathGetter;
+
+final _JSMath _jsMath = _jsMathGetter;
+
+extension type _JSMath._(JSObject _jsMath) implements JSObject {}
+
+extension _JSMathRandom on _JSMath {
+  @JS('random')
+  external double random();
+}
+
 class _SecureRandom implements Random {
   final JSUint8ArrayImpl _buffer = JSUint8ArrayImpl(8);