Fix several bugs in closure conversion.

pkg/kernel/testcases/closures/loop2.dart

+// Copyright (c) 2017, 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.
+//
+// Check that a variable declared and captured inside a loop is given a separate
+// context for each iteration of the loop, so changes to the variable in
+// subsequent iterations are not visible to closures capturing it in prior
+// iterations.
+
+void doit(int x) {
+  final int max = 10;
+  final double expectedSum = ((max - 1) * max) / 2;
+
+  int counter = 0;
+  var calls = [];
+  while (counter < max) {
+    int pos = counter;
+    calls.add(() => pos + x);
+    counter++;
+  }
+
+  double sum = 0.0;
+  for (var c in calls) sum += c();
+  if (sum != expectedSum)
+    throw new Exception("Unexpected sum = $sum != $expectedSum");
+}
+
+void main() {
+  doit(0);
+}