Implement correct semantics of Boolean Conversion

tests/language/bool_check_test.dart

+// Copyright (c) 2014, 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";
+
+bool typeChecksEnabled() {
+  try {
+    var i = 42;
+    String s = i;
+  } on TypeError catch (e) {
+    return true;
+  }
+  return false;
+}
+
+bool assertionsEnabled() {
+  try {
+    assert(false);
+    return false;
+  } on AssertionError catch (e) {
+    return true;
+  }
+  return false;
+}
+
+final bool typeChecksOn = typeChecksEnabled();
+final bool assertionsOn = assertionsEnabled();
+
+ifExpr(e) { if (e) return true; else return false; }
+bool ifNull() => ifExpr(null);
+bool ifString() => ifExpr("true");
+
+main() {
+  print("type checks: $typeChecksOn");
+  print("assertions:  $assertionsOn");
+
+  if (typeChecksOn) {
+    Expect.throws(ifNull, (e) => e is AssertionError);
+  }
+  if (assertionsOn && !typeChecksOn) {
+    Expect.throws(ifNull, (e) => e is AssertionError);
+  }
+  if (!typeChecksOn && !assertionsOn) {
+    Expect.identical(false, ifNull());
+  }
+
+  if (!typeChecksOn) {
+    Expect.identical(false, ifString());
+  }
+  if (typeChecksOn) {
+    Expect.throws(ifString, (e) => e is TypeError);
+  }
+}