Another fix incremental constants - annotations should use the old unit element.

R=brianwilkerson@google.com
BUG=

Review URL: https://codereview.chromium.org/2160673002 .

pkg/analyzer/lib/src/dart/element/element.dart

@@ -2533,7 +2533,7 @@ class ElementAnnotationImpl implements ElementAnnotation {
   /**
    * The compilation unit in which this annotation appears.
    */
-  final CompilationUnitElementImpl compilationUnit;
+  CompilationUnitElementImpl compilationUnit;
 
   /**
    * The AST of the annotation itself, cloned from the resolved AST for the

pkg/analyzer/lib/src/task/incremental_element_builder.dart

@@ -162,6 +162,12 @@ class IncrementalCompilationUnitElementBuilder {
     ConstantFinder finder = new ConstantFinder();
     oldUnit.accept(finder);
     unitConstants.addAll(finder.constantsToCompute);
+    // Update annotation constants to using the old unit element.
+    for (ConstantEvaluationTarget constant in unitConstants) {
+      if (constant is ElementAnnotationImpl) {
+        constant.compilationUnit = unitElement;
+      }
+    }
   }
 
   ClassElementDelta _processClassMembers(

pkg/analyzer/test/src/context/context_test.dart

@@ -3312,6 +3312,30 @@ int B = _A + 1;
     _assertInvalid(b, LIBRARY_ERRORS_READY);
   }
 
+  void test_sequence_add_annotation() {
+    Source a = addSource(
+        '/a.dart',
+        r'''
+const myAnnotation = const Object();
+class A {}
+''');
+    _performPendingAnalysisTasks();
+    // Add a new annotation.
+    context.setContents(
+        a,
+        r'''
+const myAnnotation = const Object();
+@myAnnotation
+class A {}
+''');
+    _assertValidForChangedLibrary(a);
+    _assertInvalid(a, LIBRARY_ERRORS_READY);
+    // Analysis is done successfully.
+    _performPendingAnalysisTasks();
+    _assertValid(a, LIBRARY_ERRORS_READY);
+    _assertValid(a, READY_RESOLVED_UNIT);
+  }
+
   void test_sequence_applyChanges_changedSource() {
     Source a = addSource(
         '/a.dart',

pkg/analyzer/test/src/task/incremental_element_builder_test.dart

@@ -1649,6 +1649,18 @@ class A {
 ''');
   }
 
+  test_update_annotation_add() {
+    _buildOldUnit(r'''
+const myAnnotation = const Object();
+foo() {}
+''');
+    _buildNewUnit(r'''
+const myAnnotation = const Object();
+@myAnnotation
+foo() {}
+''');
+  }
+
   test_update_beforeClassWithDelta_nameOffset() {
     _buildOldUnit(r'''
 class A {}
@@ -1866,6 +1878,16 @@ class _BuiltElementsValidator extends AstComparator {
           actual.getAncestor((n) => n is ClassDeclaration);
       expect(element.enclosingElement, same(classNode.element));
     }
+    // ElementAnnotationImpl must use the enclosing CompilationUnitElement.
+    if (actual is Annotation) {
+      AstNode parent = actual.parent;
+      if (parent is Declaration) {
+        ElementAnnotationImpl actualElement = actual.elementAnnotation;
+        CompilationUnitElement enclosingUnitElement =
+            parent.element.getAncestor((a) => a is CompilationUnitElement);
+        expect(actualElement.compilationUnit, same(enclosingUnitElement));
+      }
+    }
     // Identifiers like 'a.b' in 'new a.b()' might be rewritten if resolver
     // sees that 'a' is actually a class name, so 'b' is a constructor name.
     //