Create a public API for contributing assists and make assists pluggable

pkg/analysis_server/lib/edit/assist/assist_core.dart

Index: pkg/analysis_server/lib/edit/assist/assist_core.dart
diff --git a/pkg/analysis_server/lib/edit/assist/assist_core.dart b/pkg/analysis_server/lib/edit/assist/assist_core.dart
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+// Copyright (c) 2015, 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 analysis_server.edit.assist.assist_core;
+
+import 'package:analysis_server/src/protocol.dart' show SourceChange;
+import 'package:analyzer/src/generated/engine.dart';
+import 'package:analyzer/src/generated/source.dart';
+
+/**
+ * A description of a single proposed assist.
+ */
+class Assist {
+  /**
+   * An empty list of assists.
+   */
+  static const List<Assist> EMPTY_LIST = const <Assist>[];
+
+  /**
+   * A comparator that can be used to sort assists by their relevance. The most
+   * relevant assists will be sorted before assists with a lower relevance.
+   */
+  static final Comparator<Assist> SORT_BY_RELEVANCE = (Assist firstAssist,
+          Assist secondAssist) =>
+      firstAssist.kind.relevance - secondAssist.kind.relevance;
+
+  /**
+   * A description of the assist being proposed.
+   */
+  final AssistKind kind;
+
+  /**
+   * The change to be made in order to apply the assist.
+   */
+  final SourceChange change;
+
+  /**
+   * Initialize a newly created assist to have the given [kind] and [change].
+   */
+  Assist(this.kind, this.change);
+
+  @override
+  String toString() {
+    return 'Assist(kind=$kind, change=$change)';
+  }
+}
+
+/**
+ * An object used to produce assists for a specific location.
+ */
+abstract class AssistContributor {
+  /**
+   * Return a list of assists for a location in the given [source]. The location
+   * is specified by the [offset] and [length] of the selected region. The
+   * [context] can be used to get additional information that is useful for
+   * computing assists.
+   */
+  List<Assist> compute(
+      AnalysisContext context, Source source, int offset, int length);
+}
+
+/**
+ * A description of a class of assists. Instances are intended to hold the
+ * information that is common across a number of assists and to be shared by
+ * those assists.
+ */
+class AssistKind {
+  /**
+   * The name of this kind of assist, used for debugging.
+   */
+  final String name;
+
+  /**
+   * The relevance of this kind of assist for the kind of error being addressed.
+   */
+  final int relevance;
+
+  /**
+   * A human-readable description of the changes that will be applied by this
+   * kind of assist.
+   */
+  final String message;
+
+  /**
+   * Initialize a newly created kind of assist to have the given [name],
+   * [relevance] and [message].
+   */
+  const AssistKind(this.name, this.relevance, this.message);
+
+  @override
+  String toString() => name;
+}