Clear the element for imports and exports when the target doesn't exist.

pkg/analyzer/test/generated/compile_time_error_code_test.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.test.generated.compile_time_error_code_test;
 
 import 'package:analyzer/src/generated/error.dart';
 import 'package:analyzer/src/generated/parser.dart' show ParserErrorCode;
 import 'package:analyzer/src/generated/source_io.dart';
 import 'package:unittest/unittest.dart' show expect;
 
 import '../reflective_tests.dart';
 import '../utils.dart';
 import 'resolver_test.dart';
+import 'package:analyzer/src/generated/engine.dart';
 
 main() {
   initializeTestEnvironment();
   runReflectiveTests(CompileTimeErrorCodeTest);
 }
 
 @reflectiveTest
 class CompileTimeErrorCodeTest extends ResolverTestCase {
   void fail_awaitInWrongContext_sync() {
     // This test requires better error recovery than we currently have. In
@@ skipping 6012 matching lines @@
     computeLibrarySourceErrors(source);
     assertErrors(source, [CompileTimeErrorCode.URI_DOES_NOT_EXIST]);
   }
 
   void test_uriDoesNotExist_import() {
     Source source = addSource("import 'unknown.dart';");
     computeLibrarySourceErrors(source);
     assertErrors(source, [CompileTimeErrorCode.URI_DOES_NOT_EXIST]);
   }
 
-  void test_uriDoesNotExist_import_then_fixed() {
+  void test_uriDoesNotExist_import_disappears_when_fixed() {
     Source source = addSource("import 'target.dart';");
     computeLibrarySourceErrors(source);
     assertErrors(source, [CompileTimeErrorCode.URI_DOES_NOT_EXIST]);
 
     // Check that the file is represented as missing.
     Source target =
         analysisContext2.getSourcesWithFullName("/target.dart").first;
     expect(analysisContext2.getModificationStamp(target), -1);
 
     // Add an overlay in the same way as AnalysisServer.
     analysisContext2
       ..setContents(target, "")
       ..handleContentsChanged(target, null, "", true);
 
     // Make sure the error goes away.
-    // TODO(skybrian) why isn't there an unused import hint?
     computeLibrarySourceErrors(source);
     assertErrors(source, [HintCode.UNUSED_IMPORT]);
   }
 
+  void test_uriDoesNotExist_import_appears_after_deleting_target() {
+    Source test = addSource("import 'target.dart';");
+    Source target = addNamedSource("/target.dart", "");
+    computeLibrarySourceErrors(test);
+    assertErrors(test, [HintCode.UNUSED_IMPORT]);
+
+    // Remove the overlay in the same way as AnalysisServer.
+    analysisContext2.setContents(target, null);
+    ChangeSet changeSet = new ChangeSet()..removedSource(target);
+    analysisContext2.applyChanges(changeSet);
+
+    computeLibrarySourceErrors(test);
+    assertErrors(test, [CompileTimeErrorCode.URI_DOES_NOT_EXIST]);
+  }
+
   void test_uriDoesNotExist_part() {
     Source source = addSource(r'''
 library lib;
 part 'unknown.dart';''');
     computeLibrarySourceErrors(source);
     assertErrors(source, [CompileTimeErrorCode.URI_DOES_NOT_EXIST]);
   }
 
   void test_uriWithInterpolation_constant() {
     Source source = addSource("import 'stuff_\$platform.dart';");
@@ skipping 228 matching lines @@
         source, [CompileTimeErrorCode.WRONG_NUMBER_OF_PARAMETERS_FOR_OPERATOR]);
     verify([source]);
     reset();
   }
 
   void _check_wrongNumberOfParametersForOperator1(String name) {
     _check_wrongNumberOfParametersForOperator(name, "");
     _check_wrongNumberOfParametersForOperator(name, "a, b");
   }
 }