Use AbstractSingleUnitTest, so MockSdk in resynthesize tests.

pkg/analyzer/test/src/abstract_single_unit.dart

Index: pkg/analyzer/test/src/abstract_single_unit.dart
diff --git a/pkg/analyzer/test/src/abstract_single_unit.dart b/pkg/analyzer/test/src/abstract_single_unit.dart
index 1955d6e17030db5f24be4b0d07486b5d96547dd6..787ac21f4bbfa302c5f2b7a46e55d86d504d665c 100644
--- a/pkg/analyzer/test/src/abstract_single_unit.dart
+++ b/pkg/analyzer/test/src/abstract_single_unit.dart
@@ -24,9 +24,10 @@ class AbstractSingleUnitTest extends AbstractContextTest {
   CompilationUnitElement testUnitElement;
   LibraryElement testLibraryElement;
 
-  void addTestSource(String code, [Uri uri]) {
+  Source addTestSource(String code, [Uri uri]) {
     testCode = code;
     testSource = addSource(testFile, code);
+    return testSource;
   }
 
   void assertNoErrorsInSource(Source source) {