Ensures that Bigints returned to Dart are all checked by Integer::AsValidInteger.

runtime/vm/object.cc

Index: runtime/vm/object.cc
===================================================================
--- runtime/vm/object.cc	(revision 23089)
+++ runtime/vm/object.cc	(working copy)
@@ -10360,6 +10360,15 @@
 }
 
 
+RawInteger* Integer::NewFromUint64(uint64_t value, Heap::Space space) {
+  if (value > static_cast<uint64_t>(Mint::kMaxValue)) {
+    return BigintOperations::NewFromUint64(value);
+  } else {
+    return Integer::New(value);
+  }
+}
+
+
 double Integer::AsDoubleValue() const {
   UNIMPLEMENTED();
   return 0.0;