add an option to turn off errors from strong mode is-checks

pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/operations.dart

 // Copyright (c) 2015, 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.
 
 /// This library defines runtime operations on objects used by the code
 /// generator.
 part of dart._runtime;
 
 class InvocationImpl extends Invocation {
   final Symbol memberName;
@@ skipping 462 matching lines @@
 
 @JSExportName('is')
 instanceOf(obj, type) => JS(
     '',
     '''(() => {
   if ($obj == null) {
     return $type == $Null || $_isTop($type);
   }
   let result = $strongInstanceOf($obj, $type);
   if (result !== null) return result;
+  if (!$_failForWeakModeIsChecks) return false;
   let actual = $getReifiedType($obj);
   $throwStrongModeError('Strong mode is-check failure: ' +
     $typeName(actual) + ' does not soundly subtype ' +
     $typeName($type));
 })()''');
 
 @JSExportName('as')
 cast(obj, type) {
   if (JS('bool', '# == #', type, dynamic) || obj == null) return obj;
   bool result = strongInstanceOf(obj, type, true);
@@ skipping 439 matching lines @@
     name = '+' + name;
   }
   return name;
 }
 
 /// Emulates the implicit "loadLibrary" function provided by a deferred library.
 ///
 /// Libraries are not actually deferred in DDC, so this just returns a future
 /// that completes immediately.
 Future loadLibrary() => new Future.value();