Implement type inference for try/catch blocks.

pkg/front_end/testcases/inference/try_catch_promotion.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 C {}
+
+class D extends C {}
+
+class E extends StackTrace {}
+
+void test(void f()) {
+  try {
+    f();
+  } on C catch (x, y) {
+    var /*@type=C*/ x1 = x;
+    var /*@type=StackTrace*/ y1 = y;
+    if (x is D) {
+      var /*@type=D*/ x2 = /*@promotedType=D*/ x;
+    }
+    if (y is E) {
+      var /*@type=E*/ y2 = /*@promotedType=E*/ y;
+    }
+  }
+}
+
+main() {}