Use prototype inheritance to reduce number of is check properties.

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

Index: pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart b/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
index 11e028821e99b8d3e3f69d9214512a9c57bcc6b3..03358e1b2657ac5bd0ad55955a99332024f18057 100644
--- a/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
@@ -334,7 +334,7 @@ class OldEmitter implements Emitter {
     bool hasIsolateSupport = compiler.hasIsolateSupport;
     String fieldNamesProperty = FIELD_NAMES_PROPERTY_NAME;
 
-    jsAst.Expression defineClass = js('''
+    jsAst.Expression defineClass = js(r'''
         function(name, fields) {
           var accessors = [];
     
@@ -349,23 +349,24 @@ class OldEmitter implements Emitter {
             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.
@@ -441,6 +442,7 @@ class OldEmitter implements Emitter {
               // 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;
@@ -451,6 +453,7 @@ class OldEmitter implements Emitter {
                 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;