Allow 'extract index' in top-level inference.

pkg/analyzer/lib/src/task/strong/checker.dart

 // 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.
 
 // TODO(jmesserly): this was ported from package:dev_compiler, and needs to be
 // refactored to fit into analyzer.
 library analyzer.src.task.strong.checker;
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 1096 matching lines @@
         }
       }
     }
 
     if (n == null ||
         n is NullLiteral ||
         n is BooleanLiteral ||
         n is DoubleLiteral ||
         n is IntegerLiteral ||
         n is StringLiteral ||
-        n is SymbolLiteral) {
+        n is SymbolLiteral ||
+        n is IndexExpression) {
       // Nothing to validate.
     } else if (n is AwaitExpression) {
       _validateTopLevelInitializer(name, n.expression);
     } else if (n is ThrowExpression) {
       // Nothing to validate.
     } else if (n is ParenthesizedExpression) {
       _validateTopLevelInitializer(name, n.expression);
     } else if (n is ConditionalExpression) {
       _validateTopLevelInitializer(name, n.thenExpression);
       _validateTopLevelInitializer(name, n.elseExpression);
@@ skipping 436 matching lines @@
     var visited = new Set<InterfaceType>();
     do {
       visited.add(current);
       current.mixins.reversed.forEach(
           (m) => _checkIndividualOverridesFromClass(node, m, seen, true));
       _checkIndividualOverridesFromClass(node, current.superclass, seen, true);
       current = current.superclass;
     } while (!current.isObject && !visited.contains(current));
   }
 }