Add type inference for complex assignments whose LHS is an index expression.

pkg/front_end/testcases/inference/infer_assign_to_index_set_vs_get.dart

Index: pkg/front_end/testcases/inference/infer_assign_to_index_set_vs_get.dart
diff --git a/pkg/front_end/testcases/inference/infer_assign_to_index_set_vs_get.dart b/pkg/front_end/testcases/inference/infer_assign_to_index_set_vs_get.dart
new file mode 100644
index 0000000000000000000000000000000000000000..3a1f0b614e3ef9e2f5d3dc11f07ce97c3a0811ec
--- /dev/null
+++ b/pkg/front_end/testcases/inference/infer_assign_to_index_set_vs_get.dart
@@ -0,0 +1,53 @@
+// 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 {
+  C operator +(F v) => null;
+  C operator -(int i) => null;
+}
+
+class B extends A {
+  D operator +(E v) => null;
+  D operator -(int i) => null;
+}
+
+class C extends B {}
+
+class D extends C {}
+
+class E extends D {}
+
+class F extends E {}
+
+T f<T>() => null;
+
+class Test {
+  B operator [](Index i) => null;
+  void operator []=(Index i, A v) {}
+
+  void test() {
+    Test t = /*@typeArgs=Test*/ f();
+    t /*@target=Test::[]=*/ [/*@typeArgs=dynamic*/ f()] = /*@typeArgs=A*/ f();
+    t /*@target=Test::[]=*/ [/*@typeArgs=dynamic*/ f()] ??= /*@typeArgs=A*/ f();
+    t /*@target=Test::[]=*/ [
+        /*@typeArgs=dynamic*/ f()] += /*@typeArgs=dynamic*/ f();
+    --t /*@target=Test::[]=*/ [/*@typeArgs=dynamic*/ f()];
+    t /*@target=Test::[]=*/ [/*@typeArgs=dynamic*/ f()]--;
+    var /*@type=A*/ v1 = t /*@target=Test::[]=*/ [
+        /*@typeArgs=dynamic*/ f()] = /*@typeArgs=A*/ f();
+    var /*@type=A*/ v2 = t /*@target=Test::[]=*/ [
+        /*@typeArgs=dynamic*/ f()] ??= /*@typeArgs=A*/ f();
+    var /*@type=C*/ v3 = t /*@target=Test::[]=*/ [
+        /*@typeArgs=dynamic*/ f()] += /*@typeArgs=dynamic*/ f();
+    var /*@type=C*/ v4 = --t /*@target=Test::[]=*/ [/*@typeArgs=dynamic*/ f()];
+    var /*@type=A*/ v5 = t /*@target=Test::[]=*/ [/*@typeArgs=dynamic*/ f()]--;
+  }
+}
+
+main() {}