Do not analyze assert when assertions are disabled.

tests/language/assertion2_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";
+import "compiler_annotations.dart";
+
+@DontInline()
+foo() {
+  () => 42;
+  int a;
+  assert((a = 2) == 2);
+  return a;
+}
+
+main() {
+  bool isAssertEnabled = false;
+  assert(isAssertEnabled = true);
+  if (isAssertEnabled) {
+    Expect.equals(44, foo() + 42);
+  } else {
+    Expect.throws(() => foo() + 42, (e) => e is NoSuchMethodError);
+  }
+}