Only reload libraries when they may have been modified.

runtime/vm/unit_test.h

Index: runtime/vm/unit_test.h
diff --git a/runtime/vm/unit_test.h b/runtime/vm/unit_test.h
index ac7645a8ec9ae9754d7c88658e27c59edb85ac90..5a72dbb3d2cad903f3f7d4775da148806e6ba21f 100644
--- a/runtime/vm/unit_test.h
+++ b/runtime/vm/unit_test.h
@@ -314,6 +314,10 @@ class TestCase : TestCaseBase {
   // Helper function which reloads the current isolate using |script|.
   static Dart_Handle ReloadTestScript(const char* script);
 
+  // Changes the default importable test lib script.
+  static void SetImportableTestLibScript(const char* source);
+  static void RestoreImportableTestLibScript();
+
  private:
   static Dart_Isolate CreateIsolate(const uint8_t* buffer, const char* name);