improve method invocation and string interpolation suggestions

pkg/analysis_server/lib/src/services/completion/invocation_computer.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.computer.dart.invocation;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/services/completion/dart_completion_manager.dart';
 import 'package:analysis_server/src/services/completion/suggestion_builder.dart';
@@ skipping 32 matching lines @@
   Future<bool> visitConstructorName(ConstructorName node) {
     // SimpleIdentifier  PrefixedIdentifier  TypeName  ConstructorName
     Token period = node.period;
     if (period != null && period.end <= request.offset) {
       return _addNamedConstructorSuggestions(node);
     }
     return super.visitConstructorName(node);
   }
 
   @override
+  Future<bool> visitMethodInvocation(MethodInvocation node) {
+    Token period = node.period;
+    if (period == null || period.offset < request.offset) {
+      _addExpressionSuggestions(node.target);
+    }
+    return new Future.value(false);
+  }
+
+  @override
   Future<bool> visitNode(AstNode node) {
     return new Future.value(false);
   }
 
   @override
   Future<bool> visitPrefixedIdentifier(PrefixedIdentifier node) {
     if (request.offset > node.period.offset) {
       SimpleIdentifier prefix = node.prefix;
       if (prefix != null) {
         return _addElementSuggestions(prefix.bestElement);
       }
     }
     return super.visitPrefixedIdentifier(node);
   }
 
   @override
   Future<bool> visitPropertyAccess(PropertyAccess node) {
-    if (request.offset > node.offset) {
+    Token operator = node.operator;
+    if (operator != null && operator.offset < request.offset) {
       return _addExpressionSuggestions(node.realTarget);
     }
     return super.visitPropertyAccess(node);
   }
 
   @override
   Future<bool> visitSimpleIdentifier(SimpleIdentifier node) {
     return node.parent.accept(this);
   }
 
@@ skipping 72 matching lines @@
 
   @override
   Future<bool> visitVariableElement(VariableElement element) {
     DartType type = element.type;
     if (type != null) {
       ClassElementSuggestionBuilder.suggestionsFor(request, type.element);
     }
     return new Future.value(true);
   }
 }