Use prototype inheritance to reduce number of is check properties.

pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart2js.js_emitter;
 
 
 class OldEmitter implements Emitter {
   final Compiler compiler;
   final CodeEmitterTask task;
@@ skipping 316 matching lines @@
     //   print(this.x + y);
     //  },
     //  bar$2: function(t, v) {
     //   this.x = t - v;
     //  },
     // });
 
     bool hasIsolateSupport = compiler.hasIsolateSupport;
     String fieldNamesProperty = FIELD_NAMES_PROPERTY_NAME;
 
-    jsAst.Expression defineClass = js('''
+    jsAst.Expression defineClass = js(r'''
         function(name, fields) {
           var accessors = [];
     
           var str = "function " + name + "(";
           var body = "";
           if (#hasIsolateSupport) { var fieldNames = ""; }
     
           for (var i = 0; i < fields.length; i++) {
             if(i != 0) str += ", ";
     
             var field = generateAccessor(fields[i], accessors, name);
             if (#hasIsolateSupport) { fieldNames += "'" + field + "',"; }
             var parameter = "parameter_" + field;
             str += parameter;
-            body += ("this." + field + " = " + parameter + ";\\n");
+            body += ("this." + field + " = " + parameter + ";\n");
           }
-          str += ") {\\n" + body + "}\\n";
-          str += name + ".builtin\$cls=\\"" + name + "\\";\\n";
-          str += "\$desc=\$collectedClasses." + name + ";\\n";
-          str += "if(\$desc instanceof Array) \$desc = \$desc[1];\\n";
-          str += name + ".prototype = \$desc;\\n";
+          str += ") {\n" + body + "}\n";
+          str += name + ".builtin$cls=\"" + name + "\";\n";
+          str += "$desc=$collectedClasses." + name + ";\n";
+          str += "if($desc instanceof Array) $desc = \$desc[1];\n";
+          str += name + ".prototype = $desc;\n";
           if (typeof defineClass.name != "string") {
-            str += name + ".name=\\"" + name + "\\";\\n";
+            str += name + ".name=\"" + name + "\";\n";
           }
           if (#hasIsolateSupport) {
-            str += name + ".$fieldNamesProperty=[" + fieldNames + "];\\n";
+            str += name + "." + #fieldNamesProperty + "=[" + fieldNames + "];\n";

[floitsch, 2015/01/26 16:19:29]

long line.

[herhut, 2015/01/27 11:37:19]

Oops. Done.

           }
           str += accessors.join("");
     
           return str;
-        }''', { 'hasIsolateSupport': hasIsolateSupport });
+        }''', { 'hasIsolateSupport': hasIsolateSupport,
+                'fieldNamesProperty': js.string(fieldNamesProperty)});
 
     // Declare a function called "generateAccessor".  This is used in
     // defineClassFunction.
     List result = <jsAst.Node>[
         generateAccessorFunction,
         new jsAst.FunctionDeclaration(
             new jsAst.VariableDeclaration('defineClass'), defineClass) ];
 
     if (compiler.hasIncrementalSupport) {
       result.add(
@@ skipping 47 matching lines @@
   }
 
   /** Needs defineClass to be defined. */
   jsAst.Expression buildInheritFrom() {
     jsAst.Expression result = js(r'''
         function() {
           function tmp() {}
           var hasOwnProperty = Object.prototype.hasOwnProperty;
           return function (constructor, superConstructor) {
             if (superConstructor == null) {
               // TODO(21896): this test shouldn't be necessary. Without it

[floitsch, 2015/01/26 16:19:29]

you should rebase. That TODO has been fixed some time ago.

[herhut, 2015/01/27 11:37:19]

Done.

               // we have a crash in language/mixin_only_for_rti and
               // pkg/analysis_server/tool/spec/check_all_test.
               if (constructor == null) return;
 
               // Fix up the the Dart Object class' prototype.
               var prototype = constructor.prototype;
               prototype.constructor = constructor;
+              prototype.$isObject = 1;

[floitsch, 2015/01/26 16:19:29]

I think we discussed it, but I changed my mind: this should be a function, since
it is faster to check for JS engines. Same below.

This is the code and documentation that is in js_array.dart for markFixedList:

===
    // Functions are stored in the hidden class and not as properties in
    // the object. We never actually look at the value, but only want
    // to know if the property exists.
    JS('void', r'#.fixed$length = Array', list);
===

[floitsch, 2015/01/26 16:19:29]

Are you sure this works? In minified code the name of the object class might not
be "Object".

[herhut, 2015/01/27 11:37:19]

I use the constructor now, which should be a function.

[herhut, 2015/01/27 11:37:19]

Of course it does not :) This was a bit rushed yesterday. Thanks!

               return prototype;
             }
             tmp.prototype = superConstructor.prototype;
             var object = new tmp();
             var properties = constructor.prototype;
             for (var member in properties) {
               if (hasOwnProperty.call(properties, member)) {
                 object[member] = properties[member];
               }
             }
+            object["$is" + constructor.name] = 1;

[floitsch, 2015/01/26 16:19:29]

Don't just use a magic prefix.
I believe that the namer even has the prefix as property.  (same for the
$isObject above).

[herhut, 2015/01/27 11:37:19]

Done.

             object.constructor = constructor;
             constructor.prototype = object;
             return object;
           };
         }()
       ''');
     if (compiler.hasIncrementalSupport) {
       result = js(
           r'#.inheritFrom = #', [namer.accessIncrementalHelper, result]);
     }
@@ skipping 1606 matching lines @@
     for (Element element in compiler.enqueuer.codegen.newlyEnqueuedElements) {
       if (element.isInstanceMember) {
         cachedClassBuilders.remove(element.enclosingClass);
 
         nativeEmitter.cachedBuilders.remove(element.enclosingClass);
 
       }
     }
   }
 }