Update SOURCE_KIND when a missing source file appears

pkg/analyzer/test/generated/compile_time_error_code_test.dart

Index: pkg/analyzer/test/generated/compile_time_error_code_test.dart
diff --git a/pkg/analyzer/test/generated/compile_time_error_code_test.dart b/pkg/analyzer/test/generated/compile_time_error_code_test.dart
index 7c0c15adb3ea2c31093a0760f848b482c055c40e..bdaeb49f8cf33490a48bfe3c01a86c7e620f1ed9 100644
--- a/pkg/analyzer/test/generated/compile_time_error_code_test.dart
+++ b/pkg/analyzer/test/generated/compile_time_error_code_test.dart
@@ -4,9 +4,12 @@
 
 library analyzer.test.generated.compile_time_error_code_test;
 
+import 'package:analyzer/src/generated/engine.dart' show ChangeSet;
 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:analyzer/src/string_source.dart';
+import 'package:unittest/unittest.dart' show expect;
 
 import '../reflective_tests.dart';
 import '../utils.dart';
@@ -6043,6 +6046,20 @@ main() {
     assertErrors(source, [CompileTimeErrorCode.URI_DOES_NOT_EXIST]);
   }
 
+  void test_uriDoesNotExist_import_then_unused_import() {
+    Source source = addSource("import 'target.dart';");
+    computeLibrarySourceErrors(source);
+    assertErrors(source, [CompileTimeErrorCode.URI_DOES_NOT_EXIST]);
+
+    Source target =
+        analysisContext.sources.firstWhere((s) => s.fullName == "/target.dart");
+    expect(target.modificationStamp, -1);

[Brian Wilkerson, 2016/02/26 15:07:32]

I'm not 100% certain, but given that most of our tests never create any files on
disk, this might not test what you want. It might be better to use the public
API and ask the context for the modification stamp.

+    changeSource(target, "");

[Brian Wilkerson, 2016/02/26 15:07:32]

When a file that didn't exist is created on disk, do we really use changeSource?
I would have expected it to use addSource. If so, this might be the underlying
problem.

[skybrian, 2016/02/26 18:09:01]

I'm testing the case when the file hasn't been saved to disk, but we are
creating an overlay that's the target of a preexisting import statement. I
wanted to do this approximately same way as the analysis server, so it executes
the same code.

In AnalysisServer.updateContent(), first it updates the content cache directly:
String oldContents = overlayState.getContents(source);
overlayState.setContents(source, newContents);

Then for each context it will call:
List<Source> sources = context.getSourcesWithFullName(file);

A Source will exist (possibly with modificationStamp of -1) in that context if
there's any import, export, or part statement that refers to it.

Then for each Source:
context.handleContentsChanged(source, oldContents, newContents, true);

If the file didn't previously exist then I believe oldContents will be null.

[Brian Wilkerson, 2016/02/26 18:35:57]

Ok. I just didn't realize that we had clients that were creating in-memory
overrides of files that don't exist on disk.

+
+    computeLibrarySourceErrors(source);
+    assertErrors(source, [HintCode.UNUSED_IMPORT]);
+  }
+
   void test_uriDoesNotExist_part() {
     Source source = addSource(r'''
 library lib;