Fix capturing variables in optimized compilations

tests/language/regress_26453_test.dart

+// Copyright (c) 2016, 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.
+
+// The program crashed with segfault because we when we first compile foo
+// and bar we allocate all four variables (a, b, c and d) to the context.
+// When we compile foo the second time (with optimizations) we allocate
+// only c and d to the context. This happened because parser folds away
+// "${a}" and "${b}" as constant expressions when parsing bar on its own,
+// i.e. the expressions were not parsed again and thus a and b were not
+// marked as captured.
+// This caused a mismatch between a context that bar expects and that
+// the optimized version of foo produces.
+
+foo() {
+  const a = 1;
+  const b = 2;
+  var c = 3;
+  var d = 4;
+
+  bar() {
+    if ("${a}" != "1") throw "failed";
+    if ("${b}" != "2") throw "failed";
+    if ("${c}" != "3") throw "failed";
+    if ("${d}" != "4") throw "failed";
+  }
+
+  bar();
+}
+
+main() {
+  for (var i = 0; i < 50000; i++) foo();
+}