Extension method support to move us closer to a valid List implementation.

test/codegen/expect/BenchmarkBase.js

Index: test/codegen/expect/BenchmarkBase.js
diff --git a/test/codegen/expect/BenchmarkBase.js b/test/codegen/expect/BenchmarkBase.js
index 63ae85f53518446ea36ac19bd7b7f6eea0f8b35b..71be5564365528022c5691281c7b2dedb468689c 100644
--- a/test/codegen/expect/BenchmarkBase.js
+++ b/test/codegen/expect/BenchmarkBase.js
@@ -8,11 +8,11 @@ var BenchmarkBase;
       }
     }
     static listEquals(expected, actual) {
-      if (expected.length != actual.length) {
-        throw `Lists have different lengths: ${expected.length} vs ${actual.length}`;
+      if (expected[core.$length] != actual[core.$length]) {
+        throw `Lists have different lengths: ${expected[core.$length]} vs ${actual[core.$length]}`;
       }
-      for (let i = 0; dart.notNull(i) < dart.notNull(actual.length); i = dart.notNull(i) + 1) {
-        Expect.equals(expected.get(i), actual.get(i));
+      for (let i = 0; dart.notNull(i) < dart.notNull(actual[core.$length]); i = dart.notNull(i) + 1) {
+        Expect.equals(expected[core.$get](i), actual[core.$get](i));
       }
     }
     fail(message) {