Only reload libraries when they may have been modified.

runtime/vm/bit_vector.cc

Index: runtime/vm/bit_vector.cc
diff --git a/runtime/vm/bit_vector.cc b/runtime/vm/bit_vector.cc
index bf181470555da020fb5080d32cb2d8b774e1aab4..cc38df316d4cd57ce2364a2949234b820c836c84 100644
--- a/runtime/vm/bit_vector.cc
+++ b/runtime/vm/bit_vector.cc
@@ -100,6 +100,16 @@ void BitVector::Intersect(const BitVector* other) {
 }
 
 
+bool BitVector::IsEmpty() const {
+  for (intptr_t i = 0; i < data_length_; i++) {
+    if (data_[i] != 0) {
+      return false;
+    }
+  }
+  return true;
+}
+
+
 void BitVector::Print() const {
   OS::Print("[");
   for (intptr_t i = 0; i < length_; i++) {