Add List.insert.

sdk/lib/_internal/compiler/implementation/lib/js_array.dart

Index: sdk/lib/_internal/compiler/implementation/lib/js_array.dart
diff --git a/sdk/lib/_internal/compiler/implementation/lib/js_array.dart b/sdk/lib/_internal/compiler/implementation/lib/js_array.dart
index eed89a744c9eb2a2ddca385cdf4f1542e2ad64c0..b5fab30e822fb79cf6e47aeadea39331bd589f8a 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/js_array.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/js_array.dart
@@ -27,6 +27,15 @@ class JSArray<E> implements List<E> {
     return JS('var', r'#.splice(#, 1)[0]', this, index);
   }
 
+  void insertAt(int index, E value) {
+    if (index is !int) throw new ArgumentError(index);
+    if (index < 0 || index > length) {
+      throw new RangeError.value(index);
+    }
+    checkGrowable(this, 'insertAt');
+    JS('void', r'#.splice(#, 0, #)', this, index, value);
+  }
+
   E removeLast() {
     checkGrowable(this, 'removeLast');
     if (length == 0) throw new RangeError.value(-1);