add checks needed for covariant generics, and List<E>

lib/runtime/dart_runtime.js

Index: lib/runtime/dart_runtime.js
diff --git a/lib/runtime/dart_runtime.js b/lib/runtime/dart_runtime.js
index c9c21ea81135409a3a068380013d6b5474ec4d79..c529805a3c9158760ff7dc71465b0ed690eeffb3 100644
--- a/lib/runtime/dart_runtime.js
+++ b/lib/runtime/dart_runtime.js
@@ -631,6 +631,7 @@ var dart, _js_helper, _js_primitives;
   function copyProperties(to, from) {
     return copyPropertiesHelper(to, from, getOwnNamesAndSymbols(from));
   }
+  dart.copyProperties = copyProperties;
 
   /**
    * Copy symbols from the prototype of the source to destination.
@@ -641,10 +642,14 @@ var dart, _js_helper, _js_primitives;
     return copyPropertiesHelper(to.prototype, from.prototype,
         getOwnPropertySymbols(from.prototype));
   }
-
-  dart.copyProperties = copyProperties;
   dart.registerExtension = registerExtension;
 
+  function setBaseClass(derived, base) {
+    // Link the extension to the type it's extending as a base class.
+    derived.prototype.__proto__ = base.prototype;
+  }
+  dart.setBaseClass = setBaseClass;
+
   /**
    * This is called whenever a derived class needs to introduce a new field,
    * shadowing a field or getter/setter pair on its parent.
@@ -869,12 +874,18 @@ var dart, _js_helper, _js_primitives;
   }
   dart.const = constant;
 
-  /** Sets the runtime type of `obj` to be `type` */
+  /** Sets the type of `obj` to be `type` */
   function setType(obj, type) {
-    obj[_runtimeType] = type;
+    obj.__proto__ = type.prototype;
   }
   dart.setType = setType;
 
+  /** Sets the internal runtime type of `obj` to be `type` */
+  function setRuntimeType(obj, type) {
+    obj[_runtimeType] = type;
+  }
+  dart.setRuntimeType = setRuntimeType;
+
   // The following are helpers for Object methods when the receiver
   // may be null or primitive.  These should only be generated by
   // the compiler.