Force local context allocation in finally clauses (fixes #26948).

Add regression test.

R=fschneider@google.com, hausner@google.com

Review URL: https://codereview.chromium.org/2188863002 .

runtime/vm/parser.cc

@@ -9293,7 +9293,16 @@ SequenceNode* Parser::EnsureFinallyClause(
     LocalVariable* rethrow_stack_trace_var) {
   TRACE_PARSER("EnsureFinallyClause");
   ASSERT(parse || (is_async && (try_stack_ != NULL)));
-  OpenBlock();
+  // Increasing the loop level prevents the reuse of a parent context and forces
+  // the allocation of a local context to hold captured variables declared
+  // inside the finally clause. Otherwise, a captured variable gets allocated at
+  // different slots in the parent context each time the finally clause is
+  // reparsed, which is done to duplicate the ast. Since only one closure is
+  // kept due to canonicalization, it will access the correct slot in only one
+  // copy of the finally clause and the wrong slot in all others. By allocating
+  // a local context, all copies use the same slot in different local contexts.
+  // See issue #26948. This is a temporary fix until we eliminate reparsing.
+  OpenLoopBlock();
   if (parse) {
     ExpectToken(Token::kLBRACE);
   }

tests/language/regress_26948_test.dart

@@ -0,0 +1,26 @@
+// 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.
+// VMOptions=--optimization-counter-threshold=10 --no-background-compilation
+
+import 'dart:async';
+import "package:expect/expect.dart";
+
+void check(Function f) {
+  Expect.isTrue(f());
+}
+
+Future doSync() async {
+   try {
+     await 123;
+  } finally {
+    var next = 5.0;
+    check(() => next == 5.0);
+  }
+}
+
+main() async {
+  for (int i = 0; i < 20; i++) {
+    await doSync();
+  }
+}