Create a public API for contributing assists and make assists pluggable

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

Index: pkg/analysis_server/lib/edit/assist/assist_dart.dart
diff --git a/pkg/analysis_server/lib/edit/assist/assist_dart.dart b/pkg/analysis_server/lib/edit/assist/assist_dart.dart
new file mode 100644
index 0000000000000000000000000000000000000000..78166909624c7f84293e72045a2a17faf6687729
--- /dev/null
+++ b/pkg/analysis_server/lib/edit/assist/assist_dart.dart
@@ -0,0 +1,43 @@
+// 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_dart;
+
+import 'package:analysis_server/edit/assist/assist_core.dart';
+import 'package:analyzer/src/generated/ast.dart';
+import 'package:analyzer/src/generated/engine.dart';
+import 'package:analyzer/src/generated/source.dart';
+
+/**
+ * An [AssistContributor] that can be used to contribute assists for Dart
+ * files.
+ */
+abstract class DartAssistContributor extends AssistContributor {
+  @override
+  List<Assist> compute(
+      AnalysisContext context, Source source, int offset, int length) {
+    if (!AnalysisEngine.isDartFileName(source.fullName)) {
+      return Assist.EMPTY_LIST;
+    }
+    List<Source> libraries = context.getLibrariesContaining(source);
+    if (libraries.isEmpty) {
+      return Assist.EMPTY_LIST;
+    }
+    CompilationUnit unit =
+        context.resolveCompilationUnit2(source, libraries[0]);
+    if (unit == null) {
+      return Assist.EMPTY_LIST;
+    }
+    return internalComputeAssists(unit, offset, length);
+  }
+
+  /**
+   * 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> internalComputeAssists(
+      CompilationUnit unit, int offset, int length);
+}