Add support to the task model for the @required annotation

pkg/analyzer/lib/src/error/pending_error.dart

+// Copyright (c) 2016, 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.src.error.pending_error;
+
+import 'package:analyzer/dart/ast/ast.dart';
+import 'package:analyzer/dart/element/element.dart';
+import 'package:analyzer/src/generated/constant.dart';
+import 'package:analyzer/src/generated/error.dart';
+import 'package:analyzer/src/generated/source.dart';
+
+/**
+ * A pending error is an analysis error that could not be reported at the time
+ * it was discovered because some piece of information might be missing. After
+ * the information has been computed, the pending error can be converted into a
+ * real error.
+ */
+abstract class PendingError {
+  /**
+   * Create an analysis error based on the information in the pending error.
+   */
+  AnalysisError toAnalysisError();
+}
+
+/**
+ * A pending error used to compute either a [HintCode.MISSING_REQUIRED_PARAM] or
+ * [HintCode.MISSING_REQUIRED_PARAM_WITH_DETAILS] analysis error. These errors
+ * require that the value of the `@required` annotation be computed, which is
+ * not always true when the error is discovered.
+ */
+class PendingMissingRequiredParameterError implements PendingError {
+  /**
+   * The source against which the error will be reported.
+   */
+  final Source source;
+
+  /**
+   * The name of the parameter that is required.
+   */
+  final String parameterName;
+
+  /**
+   * The offset of the name of the method / function at the invocation site.
+   */
+  final int offset;
+
+  /**
+   * The length of the name of the method / function at the invocation site.
+   */
+  final int length;
+
+  /**
+   * The `@required` annotation whose value is used to compose the error message.
+   */
+  final ElementAnnotation annotation;
+
+  /**
+   * Initialize a newly created pending error.
+   */
+  PendingMissingRequiredParameterError(
+      this.source, this.parameterName, AstNode node, this.annotation)
+      : offset = node.offset,
+        length = node.length;
+
+  @override
+  AnalysisError toAnalysisError() {
+    HintCode errorCode;
+    List<String> arguments;
+    DartObject constantValue = annotation.constantValue;
+    String reason = constantValue?.getField('reason')?.toStringValue();
+    if (reason != null) {
+      errorCode = HintCode.MISSING_REQUIRED_PARAM_WITH_DETAILS;
+      arguments = [parameterName, reason];
+    } else {
+      errorCode = HintCode.MISSING_REQUIRED_PARAM;
+      arguments = [parameterName];
+    }
+    return new AnalysisError(source, offset, length, errorCode, arguments);
+  }
+}