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.direct.expect

Index: pkg/front_end/testcases/inference/infer_assign_to_index_set_vs_get.dart.direct.expect
diff --git a/pkg/front_end/testcases/inference/infer_assign_to_index_set_vs_get.dart.direct.expect b/pkg/front_end/testcases/inference/infer_assign_to_index_set_vs_get.dart.direct.expect
new file mode 100644
index 0000000000000000000000000000000000000000..5a1297adb883775da4f300eb1b82aa2165223de4
--- /dev/null
+++ b/pkg/front_end/testcases/inference/infer_assign_to_index_set_vs_get.dart.direct.expect
@@ -0,0 +1,71 @@
+library test;
+import self as self;
+import "dart:core" as core;
+
+class Index extends core::Object {
+  constructor •() → void
+    : super core::Object::•()
+    ;
+}
+class A extends core::Object {
+  constructor •() → void
+    : super core::Object::•()
+    ;
+  operator +(self::F v) → self::C
+    return null;
+  operator -(core::int i) → self::C
+    return null;
+}
+class B extends self::A {
+  constructor •() → void
+    : super self::A::•()
+    ;
+  operator +(self::E v) → self::D
+    return null;
+  operator -(core::int i) → self::D
+    return null;
+}
+class C extends self::B {
+  constructor •() → void
+    : super self::B::•()
+    ;
+}
+class D extends self::C {
+  constructor •() → void
+    : super self::C::•()
+    ;
+}
+class E extends self::D {
+  constructor •() → void
+    : super self::D::•()
+    ;
+}
+class F extends self::E {
+  constructor •() → void
+    : super self::E::•()
+    ;
+}
+class Test extends core::Object {
+  constructor •() → void
+    : super core::Object::•()
+    ;
+  operator [](self::Index i) → self::B
+    return null;
+  operator []=(self::Index i, self::A v) → void {}
+  method test() → void {
+    self::Test t = self::f();
+    t.[]=(self::f(), self::f());
+    let final self::Test #t1 = t in let final dynamic #t2 = self::f() in #t1.[](#t2).==(null) ? let final dynamic #t3 = self::f() in let final dynamic #t4 = #t1.[]=(#t2, #t3) in #t3 : null;
+    let final self::Test #t5 = t in let final dynamic #t6 = self::f() in #t5.[]=(#t6, #t5.[](#t6).+(self::f()));
+    let final self::Test #t7 = t in let final dynamic #t8 = self::f() in let final dynamic #t9 = #t7.[](#t8).-(1) in let final dynamic #t10 = #t7.[]=(#t8, #t9) in #t9;
+    let final self::Test #t11 = t in let final dynamic #t12 = self::f() in #t11.[]=(#t12, #t11.[](#t12).-(1));
+    dynamic v1 = let final self::Test #t13 = t in let final dynamic #t14 = self::f() in let final dynamic #t15 = self::f() in let final dynamic #t16 = #t13.[]=(#t14, #t15) in #t15;
+    dynamic v2 = let final self::Test #t17 = t in let final dynamic #t18 = self::f() in let final dynamic #t19 = #t17.[](#t18) in #t19.==(null) ? let final dynamic #t20 = self::f() in let final dynamic #t21 = #t17.[]=(#t18, #t20) in #t20 : #t19;
+    dynamic v3 = let final self::Test #t22 = t in let final dynamic #t23 = self::f() in let final dynamic #t24 = #t22.[](#t23).+(self::f()) in let final dynamic #t25 = #t22.[]=(#t23, #t24) in #t24;
+    dynamic v4 = let final self::Test #t26 = t in let final dynamic #t27 = self::f() in let final dynamic #t28 = #t26.[](#t27).-(1) in let final dynamic #t29 = #t26.[]=(#t27, #t28) in #t28;
+    dynamic v5 = let final self::Test #t30 = t in let final dynamic #t31 = self::f() in let final dynamic #t32 = #t30.[](#t31) in let final dynamic #t33 = #t30.[]=(#t31, #t32.-(1)) in #t32;
+  }
+}
+static method f() → dynamic
+  return null;
+static method main() → dynamic {}