Add type inference for assignment to local variables.

pkg/front_end/testcases/inference/infer_assign_to_local_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;
+
+void test1(int t) {
+  var /*@type=int*/ v1 = t = getInt();
+  var /*@type=num*/ v2 = t = getNum();
+  var /*@type=int*/ v4 = t ??= getInt();
+  var /*@type=num*/ v5 = t ??= getNum();
+  var /*@type=int*/ v7 = t += getInt();
+  var /*@type=num*/ v8 = t += getNum();
+  var /*@type=int*/ v10 = ++t;
+  var /*@type=int*/ v11 = t++;
+}
+
+void test2(num t) {
+  var /*@type=int*/ v1 = t = getInt();
+  var /*@type=num*/ v2 = t = getNum();
+  var /*@type=double*/ v3 = t = getDouble();
+  var /*@type=num*/ v4 = t ??= getInt();
+  var /*@type=num*/ v5 = t ??= getNum();
+  var /*@type=num*/ v6 = t ??= getDouble();
+  var /*@type=num*/ v7 = t += getInt();
+  var /*@type=num*/ v8 = t += getNum();
+  var /*@type=num*/ v9 = t += getDouble();
+  var /*@type=num*/ v10 = ++t;
+  var /*@type=num*/ v11 = t++;
+}
+
+void test3(double t) {
+  var /*@type=num*/ v2 = t = getNum();
+  var /*@type=double*/ v3 = t = getDouble();
+  var /*@type=num*/ v5 = t ??= getNum();
+  var /*@type=double*/ v6 = t ??= getDouble();
+  var /*@type=double*/ v7 = t += getInt();
+  var /*@type=double*/ v8 = t += getNum();
+  var /*@type=double*/ v9 = t += getDouble();
+  var /*@type=double*/ v10 = ++t;
+  var /*@type=double*/ v11 = t++;
+}
+
+main() {}