Fix analyzer tests

tests/language/string_interpolation_and_buffer_test.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 to ensure that StringBuffer and string interpolation behaves
 // the same and fail fast.
 
 import "package:expect/expect.dart";
 
 class ToStringWrapper {
   final value;
 
   ToStringWrapper(this.value);
 
   toString() => value;
 }
 
 wrap(value) => new ToStringWrapper(value);
 
+final bool checkedMode = computeCheckedMode();
+bool computeCheckedMode() {
+  try {
+    int x = "foo";
+  } on Error {
+    return true;
+  }
+  return false;
+}
+
 main() {
-  bool checkedMode = false;
-  assert(checkedMode = true);
   interpolate(object) {
     var result;
     if (checkedMode && object != null) {
       try {
         result = '${wrap(object)}';
       } on TypeError {
         return 'Error';
       } on ArgumentError {
         return 'Error';  // Checked mode.
       }
@@ skipping 64 matching lines @@
   Expect.equals('Error', buffer([1]));
   Expect.equals('Error', buffer(new Object()));
 
   Expect.equals('Error', initBuffer(null));
   Expect.equals('Success', initBuffer(""));
   Expect.equals('Success', initBuffer("string"));
   Expect.equals('Error', initBuffer([]));
   Expect.equals('Error', initBuffer([1]));
   Expect.equals('Error', initBuffer(new Object()));
 }