Enable is and as checks on non-ground types

lib/runtime/dart_utils.js

 // 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 a set of general javascript utilities for us
  * by the Dart runtime.
 */
 
 var dart_utils =
   typeof module != "undefined" && module.exports || {};
 
 (function (dart_utils) {
   'use strict';
 
   const defineProperty = Object.defineProperty;
   const getOwnPropertyDescriptor = Object.getOwnPropertyDescriptor;
   const getOwnPropertyNames = Object.getOwnPropertyNames;
   const getOwnPropertySymbols = Object.getOwnPropertySymbols;
 
   const hasOwnProperty = Object.prototype.hasOwnProperty;
 
   const slice = [].slice;
 
+  class StrongModeError extends Error {
+    constructor(message) {
+      super(message);
+    }
+  }
+
+  /** This error indicates a strong mode specific failure.
+   */
+  function throwStrongModeError(message) {
+    throw new StrongModeError(message);
+  }
+  dart_utils.throwStrongModeError = throwStrongModeError;
 
   /** This error indicates a bug in the runtime or the compiler.
    */
-  function throwError(message) {
+  function throwInternalError(message) {
     throw Error(message);
   }
-  dart_utils.throwError = throwError;
+  dart_utils.throwInternalError = throwInternalError;
 
   function assert(condition) {
-    if (!condition) throwError("The compiler is broken: failed assert");
+    if (!condition) throwInternalError("The compiler is broken: failed assert");
   }
   dart_utils.assert = assert;
 
   function getOwnNamesAndSymbols(obj) {
     return getOwnPropertyNames(obj).concat(getOwnPropertySymbols(obj));
   }
   dart_utils.getOwnNamesAndSymbols = getOwnNamesAndSymbols;
 
   function safeGetOwnProperty(obj, name) {
     let desc = getOwnPropertyDescriptor(obj, name);
     if (desc) return desc.value;
   }
   dart_utils.safeGetOwnProperty = safeGetOwnProperty;
 
   /**
    * Defines a lazy property.
    * After initial get or set, it will replace itself with a value property.
    */
   // TODO(jmesserly): reusing descriptor objects has been shown to improve
   // performance in other projects (e.g. webcomponents.js ShadowDOM polyfill).
   function defineLazyProperty(to, name, desc) {
     let init = desc.get;
     let value = null;
 
     function lazySetter(x) {
       init = null;
       value = x;
     }
     function circularInitError() {
-      throwError('circular initialization for field ' + name);
+      throwInternalError('circular initialization for field ' + name);
     }
     function lazyGetter() {
       if (init == null) return value;
 
       // Compute and store the value, guarding against reentry.
       let f = init;
       init = circularInitError;
       lazySetter(f());
       return value;
     }
@@ skipping 40 matching lines @@
     }
     if (hide != void 0) {
       var hideMap = new Set(hide);
       show = show.filter((k) => !hideMap.has(k));
     }
     return copyTheseProperties(to, from, show);
   }
   dart_utils.export = export_;
 
 })(dart_utils);