Allow LetCont to bind multiple continuations.

tests/compiler/dart2js/js_backend_cps_ir_basic_test.dart

 // Copyright (c) 2014, 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=-DUSE_CPS_IR=true
 
 // Tests for basic functionality.
 
 library basic_tests;
 
 import 'js_backend_cps_ir.dart';
 
 const List<TestEntry> tests = const [
   const TestEntry("""
 foo(a, [b = "b"]) => b;
 bar(a, {b: "b", c: "c"}) => c;
 main() {
   foo(0);
-  foo(0, 1);
-  bar(0);
-  bar(0, b: 1);
-  bar(0, c: 1);
-  bar(0, b: 1, c: 2);
+  foo(1, 2);
+  bar(3);
+  bar(4, b: 5);
+  bar(6, c: 7);
+  bar(8, b: 9, c: 10);
 }
 """,
 """
 function() {
   V.foo(0, "b");
-  V.foo(0, 1);
-  V.bar(0, "b", "c");
-  V.bar(0, 1, "c");
-  V.bar(0, "b", 1);
-  V.bar(0, 1, 2);
+  V.foo(1, 2);
+  V.bar(3, "b", "c");
+  V.bar(4, 5, "c");
+  V.bar(6, "b", 7);
+  V.bar(8, 9, 10);
   return null;
 }"""),
   const TestEntry(
   """
 foo(a) {
   return a;
 }
 main() {
   var a = 10;
   var b = 1;
@@ skipping 57 matching lines @@
 function() {
   P.print(P.DateTime$now().isBefore$1(P.DateTime$now()));
   return null;
 }"""),
 ];
 
 
 void main() {
   runTests(tests);
 }