Fix a bug in the type inferrer.

tests/language/inferrer_constructor4_test.dart

+// Copyright (c) 2013, 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.
+
+import "package:expect/expect.dart";
+
+escape(object) {
+  print(object.field + 42);
+}
+
+class A {
+  A() {
+    escape(this);
+  }
+}
+
+class B extends A {
+  var field;
+  B() {
+    field = 42;
+  }
+}
+
+main() {
+  Expect.throws(() => new B(), (e) => e is NoSuchMethodError);
+}