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

pkg/front_end/testcases/inference/infer_assign_to_index_this.dart

+// 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.
+
+/*@testedFeatures=inference*/
+library test;
+
+class Index {}
+
+class A {}
+
+class B extends A {
+  A operator +(C v) => null;
+  B operator -(int i) => null;
+  B operator *(B v) => null;
+  C operator &(A v) => null;
+}
+
+class C extends B {}
+
+T f<T>() => null;
+
+class Test {
+  B operator [](Index i) => null;
+  void operator []=(Index i, B v) {}
+
+  void test() {
+    this /*@target=Test::[]=*/ [
+        /*@typeArgs=dynamic*/ f()] = /*@typeArgs=B*/ f();
+    this /*@target=Test::[]=*/ [
+        /*@typeArgs=dynamic*/ f()] ??= /*@typeArgs=B*/ f();
+    this /*@target=Test::[]=*/ [
+        /*@typeArgs=dynamic*/ f()] += /*@typeArgs=dynamic*/ f();
+    this /*@target=Test::[]=*/ [
+        /*@typeArgs=dynamic*/ f()] *= /*@typeArgs=dynamic*/ f();
+    this /*@target=Test::[]=*/ [
+        /*@typeArgs=dynamic*/ f()] &= /*@typeArgs=dynamic*/ f();
+    --this /*@target=Test::[]=*/ [/*@typeArgs=dynamic*/ f()];
+    this /*@target=Test::[]=*/ [/*@typeArgs=dynamic*/ f()]--;
+    var /*@type=B*/ v1 = this /*@target=Test::[]=*/ [
+        /*@typeArgs=dynamic*/ f()] = /*@typeArgs=B*/ f();
+    var /*@type=B*/ v2 = this /*@target=Test::[]=*/ [
+        /*@typeArgs=dynamic*/ f()] ??= /*@typeArgs=B*/ f();
+    var /*@type=A*/ v3 = this /*@target=Test::[]=*/ [
+        /*@typeArgs=dynamic*/ f()] += /*@typeArgs=dynamic*/ f();
+    var /*@type=B*/ v4 = this /*@target=Test::[]=*/ [
+        /*@typeArgs=dynamic*/ f()] *= /*@typeArgs=dynamic*/ f();
+    var /*@type=C*/ v5 = this /*@target=Test::[]=*/ [
+        /*@typeArgs=dynamic*/ f()] &= /*@typeArgs=dynamic*/ f();
+    var /*@type=B*/ v6 =
+        --this /*@target=Test::[]=*/ [/*@typeArgs=dynamic*/ f()];
+    var /*@type=B*/ v7 =
+        this /*@target=Test::[]=*/ [/*@typeArgs=dynamic*/ f()]--;
+  }
+}
+
+main() {}