[analyzer] Add LSP integration test for diagnostic context messages

See #44907.

Change-Id: I780e584a0e624aac232439d3f87cf1fb1284ef9f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/193800
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

pkg/analysis_server/test/integration/lsp_server/diagnostic_test.dart

@@ -17,6 +17,34 @@ void main() {
 
 @reflectiveTest
 class DiagnosticTest extends AbstractLspAnalysisServerIntegrationTest {
+  Future<void> test_contextMessage() async {
+    const content = '''
+void f() {
+  x = 0;
+  int [[x]] = 1;
+  print(x);
+}
+''';
+    newFile(mainFilePath, content: withoutMarkers(content));
+
+    final diagnosticsUpdate = waitForDiagnostics(mainFileUri);
+    await initialize();
+    final diagnostics = await diagnosticsUpdate;
+
+    expect(diagnostics, hasLength(1));
+    final diagnostic = diagnostics.first;
+    expect(
+        diagnostic.message,
+        startsWith(
+            "Local variable 'x' can't be referenced before it is declared"));
+
+    expect(diagnostic.relatedInformation, hasLength(1));
+    final relatedInfo = diagnostic.relatedInformation.first;
+    expect(relatedInfo.message, equals("The declaration of 'x' is here."));
+    expect(relatedInfo.location.uri, equals('$mainFileUri'));
+    expect(relatedInfo.location.range, equals(rangeFromMarkers(content)));
+  }
+
   Future<void> test_initialAnalysis() async {
     newFile(mainFilePath, content: 'String a = 1;');