Fix a couple type bugs in dart html and what seems like strange type choices in dart:core

sdk/lib/_internal/compiler/js_lib/js_array.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.
 
 part of _interceptors;
 
 /**
  * The interceptor class for [List]. The compiler recognizes this
  * class as an interceptor, and changes references to [:this:] to
  * actually use the receiver of the method, which is generated as an extra
@@ skipping 195 matching lines @@
   }
 
   E reduce(E combine(E value, E element)) {
     return IterableMixinWorkaround.reduce(this, combine);
   }
 
   fold(initialValue, combine(previousValue, E element)) {
     return IterableMixinWorkaround.fold(this, initialValue, combine);
   }
 
-  dynamic firstWhere(bool test(E value), {Object orElse()}) {
+  E firstWhere(bool test(E value), {E orElse()}) {
     return IterableMixinWorkaround.firstWhere(this, test, orElse);
   }
 
-  dynamic lastWhere(bool test(E value), {Object orElse()}) {
+  E lastWhere(bool test(E value), {E orElse()}) {
     return IterableMixinWorkaround.lastWhereList(this, test, orElse);
   }
 
   E singleWhere(bool test(E value)) {
     return IterableMixinWorkaround.singleWhere(this, test);
   }
 
   E elementAt(int index) {
     return this[index];
   }
@@ skipping 149 matching lines @@
 }
 
 /**
  * Dummy subclasses that allow the backend to track more precise
  * information about arrays through their type. The CPA type inference
  * relies on the fact that these classes do not override [] nor []=.
  */
 class JSMutableArray<E> extends JSArray<E> implements JSMutableIndexable {}
 class JSFixedArray<E> extends JSMutableArray<E> {}
 class JSExtendableArray<E> extends JSMutableArray<E> {}