Add type inference for assignments to `this[...]`

pkg/front_end/lib/src/fasta/kernel/fasta_accessors.dart

 // Copyright (c) 2016, 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 fasta.fasta_accessors;
 
 import 'package:front_end/src/fasta/kernel/kernel_shadow_ast.dart'
     show
         KernelArguments,
         KernelComplexAssignment,
@@ skipping 781 matching lines @@
 
   String get plainNameForWrite => "[]=";
 
   Expression doInvocation(int offset, Arguments arguments) {
     return helper.buildMethodInvocation(
         buildSimpleRead(), callName, arguments, offset,
         isImplicitCall: true);
   }
 
   toString() => "ThisIndexAccessor()";
+
+  @override
+  KernelComplexAssignment startComplexAssignment(Expression rhs) =>
+      new KernelIndexAssign(null, index, rhs);
 }
 
 class SuperIndexAccessor extends kernel.SuperIndexAccessor with FastaAccessor {
   SuperIndexAccessor(BuilderHelper helper, Token token, Expression index,
       Member getter, Member setter)
       : super(helper, index, getter, setter, token);
 
   String get plainNameForRead => "[]";
 
   String get plainNameForWrite => "[]=";
@@ skipping 225 matching lines @@
     offset ??= offsetForToken(this.token);
     return helper.throwNoSuchMethodError(new NullLiteral()..fileOffset = offset,
         plainNameForRead, arguments, offset,
         isGetter: isGetter, isSetter: isSetter);
   }
 }
 
 bool isFieldOrGetter(Member member) {
   return member is Field || (member is Procedure && member.isGetter);
 }