Reapply "This change makes it easier to put type parameters on JavaScript Arrays."

tests/compiler/dart2js/mock_compiler.dart

Index: tests/compiler/dart2js/mock_compiler.dart
diff --git a/tests/compiler/dart2js/mock_compiler.dart b/tests/compiler/dart2js/mock_compiler.dart
index 8f1963ccbeb95547f2e2b244a9615bbe9afd4e48..15ce07cca3ca373a76b382ca507e577192e28131 100644
--- a/tests/compiler/dart2js/mock_compiler.dart
+++ b/tests/compiler/dart2js/mock_compiler.dart
@@ -102,7 +102,9 @@ const String DEFAULT_INTERCEPTORSLIB = r'''
     operator[](index);
   }
   abstract class JSMutableIndexable extends JSIndexable {}
-  class JSArray extends Interceptor implements List, JSIndexable {
+  class JSArray<E> extends Interceptor implements List<E>, JSIndexable {
+    JSArray();
+    factory JSArray.typed(a) => a;
     var length;
     operator[](index) => this[index];
     operator[]=(index, value) { this[index] = value; }