Add type inference for assignment to local variables.

pkg/front_end/testcases/inference/infer_assign_to_local.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 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;
+
+void test() {
+  B local;
+  local = /*@typeArgs=B*/ f();
+  local ??= /*@typeArgs=B*/ f();
+  local += /*@typeArgs=dynamic*/ f();
+  local *= /*@typeArgs=dynamic*/ f();
+  local &= /*@typeArgs=dynamic*/ f();
+  --local;
+  local--;
+  var /*@type=B*/ v1 = local = /*@typeArgs=B*/ f();
+  var /*@type=B*/ v2 = local ??= /*@typeArgs=B*/ f();
+  var /*@type=A*/ v3 = local += /*@typeArgs=dynamic*/ f();
+  var /*@type=B*/ v4 = local *= /*@typeArgs=dynamic*/ f();
+  var /*@type=C*/ v5 = local &= /*@typeArgs=dynamic*/ f();
+  var /*@type=B*/ v6 = --local;
+  var /*@type=B*/ v7 = local--;
+}
+
+main() {}