Add support for execution of constructor body

pkg/kernel/testcases/interpreter/constructor_test.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.
+
+library constructor_test;
+
+void main() {
+  var a = new A();
+  print(a.foo);
+  print('******************');
+
+  var b = new B();
+  print(b.foo);
+  print(b.bar);
+}
+
+class A {
+  String foo = fieldInitializer(0, 'A.foo');
+
+  A() : foo = fieldInitializer(1, 'A.foo') {
+    foo = fieldInitializer(2, 'A.foo');
+  }
+}
+
+class B extends A {
+  String bar = fieldInitializer(0, 'B.bar');
+
+  B() : bar = fieldInitializer(1, 'B.bar') {
+    bar = fieldInitializer(2, 'B.bar');
+  }
+}
+
+String fieldInitializer(int f, String s) {
+  print('$s: $f');
+  return '$s: $f';
+}