Add test for native extensions that copies the extension to a test directory and runs it.

tools/testing/dart/multitest.dart

Index: tools/testing/dart/multitest.dart
diff --git a/tools/testing/dart/multitest.dart b/tools/testing/dart/multitest.dart
index 5170ac1f46a5fe553e78033548179ce4f1c67de5..4e1b71665163fd867693d6f6ef7bea150664b3f2 100644
--- a/tools/testing/dart/multitest.dart
+++ b/tools/testing/dart/multitest.dart
@@ -160,9 +160,9 @@ Set<String> _findAllRelativeImports(String topLibrary) {
   String libraryDir = topLibrary.substring(0, end);
 
   // Matches #import( or #source( followed by " or ' followed by anything
-  // except dart: or /, at the beginning of a line.
-  RegExp relativeImportRegExp =
-      const RegExp('^#(import|source)[(]["\'](?!(dart:|/))([^"\']*)["\']');
+  // except dart:, dart-ext: or /, at the beginning of a line.
+  RegExp relativeImportRegExp = const RegExp(
+      '^#(import|source)[(]["\'](?!(dart:|dart-ext:|/))([^"\']*)["\']');
   while (!toSearch.isEmpty()) {
     var thisPass = toSearch;
     toSearch = new HashSet<String>();