Fix a crash in the compiler.

tests/language/phi_merge_test.dart

+// Copyright (c) 2013, 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.
+
+// Regression test for dart2js that used to crash on this code.
+
+class A {
+  operator[]=(index, value) {
+    switch (value) {
+      case 42: break;
+      case 43: break;
+    }
+  }
+}
+
+main() {
+  // Make [a] a phi.
+  var a;
+  if (true) {
+    a = new A();
+  } else {
+    a = new A();
+  }
+  // `A[]=` being inlined, the compiler was confused when merging the
+  // phis after the switch.
+  a[0] = 42;
+
+  // Use [a] to provoke the crash.
+  print(a);
+}