Enable is and as checks on non-ground types

lib/runtime/dart_library.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 file defines the module loader for the dart runtime.
 */
 
 var dart_library =
   typeof module != "undefined" && module.exports || {};
 
@@ skipping 30 matching lines @@
 
     loadLazyImports(pendingSet) {
       return this.handleImports(pendingSet, (lib) => lib.load());
     }
 
     handleImports(list, handler) {
       let results = [];
       for (let name of list) {
         let lib = libraries[name];
         if (!lib) {
-          dart_utils.throwError('Library not available: ' + name);
+          dart_utils.throwInternalError('Library not available: ' + name);
         }
         results.push(handler(lib));
       }
       return results;
     }
 
     load(inheritedPendingSet) {
       // Check for cycles
       if (this._state == LibraryLoader.LOADING) {
-        dart_utils.throwError('Circular dependence on library: '
+        dart_utils.throwInternalError('Circular dependence on library: '
                               + this._name);
       } else if (this._state >= LibraryLoader.LOADED) {
         return this._library;
       }
       this._state = LibraryLoader.LOADING;
 
       // Handle imports and record lazy imports
       let pendingSet = inheritedPendingSet ? inheritedPendingSet : new Set();
       let args = this.loadImports(pendingSet);
       args = args.concat(this.deferLazyImports(pendingSet));
@@ skipping 26 matching lines @@
 
   function library(name, defaultValue, imports, lazyImports, loader) {
     return libraries[name] =
       new LibraryLoader(name, defaultValue, imports, lazyImports, loader);
   }
   dart_library.library = library;
 
   function import_(libraryName) {
     bootstrap();
     let loader = libraries[libraryName];
-    if (!loader) dart_utils.throwError('Library not found: ' + libraryName);
+    // TODO(vsm): A user might call this directly from JS (as we do in tests).
+    // We may want a different error type.
+    if (!loader) dart_utils.throwInternalError('Library not found: ' + libraryName);
     return loader.load();
   }
   dart_library.import = import_;
 
   function start(libraryName) {
     let library = import_(libraryName);
     let _isolate_helper = import_('dart/_isolate_helper');
     _isolate_helper.startRootIsolate(library.main, []);
   }
   dart_library.start = start;
@@ skipping 15 matching lines @@
     // See: https://github.com/dart-lang/dev_compiler/issues/173
     if (typeof NodeList !== "undefined") {
       NodeList.prototype.get = function(i) { return this[i]; };
       NamedNodeMap.prototype.get = function(i) { return this[i]; };
       DOMTokenList.prototype.get = function(i) { return this[i]; };
       HTMLCollection.prototype.get = function(i) { return this[i]; };
     }
   }
 
 })(dart_library);