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

tests/utils/dummy_compiler_test.dart

Index: tests/utils/dummy_compiler_test.dart
diff --git a/tests/utils/dummy_compiler_test.dart b/tests/utils/dummy_compiler_test.dart
index 4c0a4aa9d1d8c268d9a48098f250ae5c8dcbb49c..aad4e497c04d4a6f7348b50f0d2ab305406c5a6f 100644
--- a/tests/utils/dummy_compiler_test.dart
+++ b/tests/utils/dummy_compiler_test.dart
@@ -32,7 +32,7 @@ class int {}
 class double{}
 class String{}
 class Function{}
-class List {}
+class List<E> {}
 class Map {}
 class Closure {}
 class BoundClosure {}
@@ -57,7 +57,9 @@ class JSIndexable {
   get length;
 }
 class JSMutableIndexable {}
-class JSArray implements JSIndexable {
+class JSArray<E> implements JSIndexable {
+  JSArray() {}
+  factory JSArray.typed(a) => a;
   var removeLast;
   var add;
 }