Implement type inference for assignments to a static variable.

pkg/front_end/testcases/inference/infer_assign_to_static_upwards.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;
+
+int getInt() => 0;
+num getNum() => 0;
+double getDouble() => 0.0;
+
+int topLevelInt;
+num topLevelNum;
+double topLevelDouble;
+
+void test1() {
+  var /*@type=int*/ v1 = topLevelInt = getInt();
+  var /*@type=num*/ v2 = topLevelInt = getNum();
+  var /*@type=int*/ v4 = topLevelInt ??= getInt();
+  var /*@type=num*/ v5 = topLevelInt ??= getNum();
+  var /*@type=int*/ v7 = topLevelInt += getInt();
+  var /*@type=num*/ v8 = topLevelInt += getNum();
+  var /*@type=int*/ v10 = ++topLevelInt;
+  var /*@type=int*/ v11 = topLevelInt++;
+}
+
+void test2() {
+  var /*@type=int*/ v1 = topLevelNum = getInt();
+  var /*@type=num*/ v2 = topLevelNum = getNum();
+  var /*@type=double*/ v3 = topLevelNum = getDouble();
+  var /*@type=num*/ v4 = topLevelNum ??= getInt();
+  var /*@type=num*/ v5 = topLevelNum ??= getNum();
+  var /*@type=num*/ v6 = topLevelNum ??= getDouble();
+  var /*@type=num*/ v7 = topLevelNum += getInt();
+  var /*@type=num*/ v8 = topLevelNum += getNum();
+  var /*@type=num*/ v9 = topLevelNum += getDouble();
+  var /*@type=num*/ v10 = ++topLevelNum;
+  var /*@type=num*/ v11 = topLevelNum++;
+}
+
+void test3() {
+  var /*@type=num*/ v2 = topLevelDouble = getNum();
+  var /*@type=double*/ v3 = topLevelDouble = getDouble();
+  var /*@type=num*/ v5 = topLevelDouble ??= getNum();
+  var /*@type=double*/ v6 = topLevelDouble ??= getDouble();
+  var /*@type=double*/ v7 = topLevelDouble += getInt();
+  var /*@type=double*/ v8 = topLevelDouble += getNum();
+  var /*@type=double*/ v9 = topLevelDouble += getDouble();
+  var /*@type=double*/ v10 = ++topLevelDouble;
+  var /*@type=double*/ v11 = topLevelDouble++;
+}
+
+main() {}