dart2js cps: Handle checks against mutable and extendable lists.

pkg/compiler/lib/src/js_backend/codegen/codegen.dart

Index: pkg/compiler/lib/src/js_backend/codegen/codegen.dart
diff --git a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
index d7453dbf44427ebe325ca5bd2e62d4ffeb7dfa05..fcddc2bd2bbb5bc4dc1ebfaf87d08cc746282d73 100644
--- a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
+++ b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
@@ -337,6 +337,14 @@ class CodeGenerator extends tree_ir.StatementVisitor
       glue.registerIsCheck(type, registry);
       ClassElement clazz = type.element;
 
+      // Handle some special checks against classes that exist only in
+      // the compile-time class hierarchy, not at runtime.
+      if (clazz == glue.jsExtendableArrayClass) {
+        return js.js(r'!#.fixed$length', <js.Expression>[value]);
+      } else if (clazz == glue.jsMutableArrayClass) {
+        return js.js(r'!#.immutable$list', <js.Expression>[value]);
+      }
+
       // We use one of the two helpers:
       //
       //     checkSubtype(value, $isT, typeArgs, $asT)