Fix static type analysis of closure calls (issue 3959).

tests/language/closure_type_test.dart

+// Copyright (c) 2012, 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.
+// Dart test for a closure result type test that cannot be eliminated at compile
+// time.
+
+isCheckedMode() {
+  try {
+    var i = 1;
+    String s = i;
+    return false;
+  } catch(var e) {
+    return true;
+  }
+}
+
+
+void test(int func(int value), int value) {
+  bool got_type_error = false;
+  try {
+    // Because of function subtyping rules, the static return type of a closure
+    // call cannot be relied upon for static type analysis. For example, a
+    // function returning Dynamic (function 'root') can be assigned to a closure
+    // variable declared to return int (closure 'func') and may actually return
+    // a double at run-time.
+    // Therefore, eliminating the run-time type check would be wrong.
+    int x = func(value);
+    Expect.equals(value, x * x);
+  } catch (TypeError error) {
+    got_type_error = true;
+  }
+  // Type error expected in checked mode only.
+  Expect.isTrue(got_type_error == isCheckedMode());
+}
+
+root(x) => Math.sqrt(x);
+
+main() => test(root, 4);