Add support for requesting units from resynthesizer.

pkg/analyzer/test/src/summary/resynthesize_test.dart

Index: pkg/analyzer/test/src/summary/resynthesize_test.dart
diff --git a/pkg/analyzer/test/src/summary/resynthesize_test.dart b/pkg/analyzer/test/src/summary/resynthesize_test.dart
index 702107052ea93ff18fd952ca4a44cd4a50942d53..b75499d4152b5a8ec5f2f99d1106f6012e7a697d 100644
--- a/pkg/analyzer/test/src/summary/resynthesize_test.dart
+++ b/pkg/analyzer/test/src/summary/resynthesize_test.dart
@@ -2535,6 +2535,14 @@ f() {}''');
     comparePropertyAccessorElements(resynthesized, original, 'C.setter f');
   }
 
+  test_getElement_unit() {
+    Source source = addSource('class C { f() {} }');
+    CompilationUnitElement original = resolve2(source).definingCompilationUnit;
+    expect(original, isNotNull);
+    CompilationUnitElement resynthesized = validateGetElement(original);
+    compareCompilationUnitElements(resynthesized, original);
+  }
+
   test_getter_documented() {
     checkLibrary('''
 // Extra comment so doc comment offset != 0