Fix a crash when optimizing instance field increment: if the increment operation gets interrupted b…

tests/language/src/ExceptionInIncrement.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.
+// Test throws exception in the middle of the increment operation, the setter
+// part of the instance field increment never completes.
+
+main() {
+  var a = new A();
+  a.field = new A();
+  for (int i = 0; i < 4000; i++) {
+    try {
+      a.foo(i);
+    } catch (var e) {}
+  }
+}
+
+class A {
+  var field;
+  foo(i) {
+    field++;  // throw exception
+  }
+}