Add support code to make fixes easier in plugins

pkg/analyzer_plugin/lib/utilities/fixes/fix_contributor_mixin.dart

Index: pkg/analyzer_plugin/lib/utilities/fixes/fix_contributor_mixin.dart
diff --git a/pkg/analyzer_plugin/lib/utilities/fixes/fix_contributor_mixin.dart b/pkg/analyzer_plugin/lib/utilities/fixes/fix_contributor_mixin.dart
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index 0000000000000000000000000000000000000000..89d5a8614237ec2e1322fc78c2ef3c438a0668d8
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+// Copyright (c) 2017, 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.
+
+import 'package:analyzer/error/error.dart';
+import 'package:analyzer/src/generated/java_core.dart';
+import 'package:analyzer_plugin/protocol/protocol_common.dart'
+    hide AnalysisError;
+import 'package:analyzer_plugin/protocol/protocol_generated.dart';
+import 'package:analyzer_plugin/utilities/change_builder/change_builder_core.dart';
+import 'package:analyzer_plugin/utilities/fixes/fixes.dart';
+
+/**
+ * A partial implementation of a [FixContributor] that iterates over the list of
+ * errors and provides a utility method to make it easier to add fixes.
+ *
+ * Clients may not extend or implement this class, but are allowed to use it as
+ * a mix-in when creating a subclass of [FixContributor].
+ */
+abstract class FixContributorMixin implements FixContributor {
+  /**
+   * The request that specifies the fixes that are to be built.
+   */
+  DartFixesRequest request;
+
+  /**
+   * The collector to which fixes should be added.
+   */
+  FixCollector collector;
+
+  /**
+   * Add a fix for the given [error]. Use the [kind] of the fix to get the
+   * message and priority, and use the change [builder] to get the edits that
+   * comprise the fix. If the message has parameters, then use the list of
+   * [args] to populate the message.
+   */
+  void addFix(AnalysisError error, FixKind kind, ChangeBuilder builder,
+      {List<Object> args: null}) {
+    SourceChange change = builder.sourceChange;
+    if (change.edits.isEmpty) {
+      return;
+    }
+    change.message = formatList(kind.message, args);
+    collector.addFix(error,
+        new PrioritizedSourceChange(kind.priority, builder.sourceChange));
+  }
+
+  @override
+  void computeFixes(DartFixesRequest request, FixCollector collector) {
+    this.request = request;
+    this.collector = collector;
+    try {
+      for (AnalysisError error in request.errorsToFix) {
+        computeFixesForError(error);
+      }
+    } finally {
+      this.request = null;
+      this.collector = null;
+    }
+  }
+
+  /**
+   * Compute the fixes that are appropriate for the given [error] and add them
+   * to the fix [collector].
+   */
+  void computeFixesForError(AnalysisError error);
+}