suggest async for closures

pkg/analysis_server/lib/src/services/completion/keyword_contributor.dart

 // Copyright (c) 2014, 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 services.completion.contributor.dart.keyword;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/protocol.dart';
 import 'package:analysis_server/src/services/completion/dart_completion_manager.dart';
@@ skipping 34 matching lines @@
   @override
   visitArgumentList(ArgumentList node) {
     if (entity == node.rightParenthesis ||
         (entity is SimpleIdentifier && node.arguments.contains(entity))) {
       _addExpressionKeywords(node);
     }
   }
 
   @override
   visitBlock(Block node) {
+    if (entity is ExpressionStatement) {
+      Expression expression = (entity as ExpressionStatement).expression;
+      if (expression is SimpleIdentifier) {
+        Token token = expression.token;
+        Token previous = token.previous;
+        if (previous.isSynthetic) {
+          previous = previous.previous;
+        }
+        Token next = token.next;
+        if (next.isSynthetic) {
+          next = next.next;
+        }
+        if (previous.lexeme == ')' && next.lexeme == '{') {
+          _addSuggestion2(ASYNC);
+        }
+      }
+    }
     _addStatementKeywords(node);
   }
 
   @override
   visitClassDeclaration(ClassDeclaration node) {
     // Don't suggest class name
     if (entity == node.name) {
       return;
     }
     if (entity == node.rightBracket) {
@@ skipping 79 matching lines @@
     AstNode constructorDecl =
         node.getAncestor((p) => p is ConstructorDeclaration);
     if (constructorDecl != null) {
       _addSuggestions([Keyword.THIS]);
     }
   }
 
   @override
   visitFunctionExpression(FunctionExpression node) {
     if (entity == node.body) {
-      _addSuggestion2(ASYNC, relevance: DART_RELEVANCE_HIGH);
+      if (!node.body.isAsynchronous) {
+        _addSuggestion2(ASYNC, relevance: DART_RELEVANCE_HIGH);
+      }
       if (node.body is EmptyFunctionBody &&
           node.parent is FunctionDeclaration &&
           node.parent.parent is CompilationUnit) {
         _addCompilationUnitKeywords();
       }
     }
   }
 
   @override
   visitIfStatement(IfStatement node) {
@@ skipping 206 matching lines @@
         return false;
       }
       AstNode parent = body.parent;
       if (parent is ConstructorDeclaration || parent is MethodDeclaration) {
         return true;
       }
       node = parent;
     }
   }
 }