dart2js: --fast-startup: cache prototypes in local in tear-off setup

pkg/compiler/lib/src/js_emitter/startup_emitter/fragment_emitter.dart

 // Copyright (c) 2015, 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.startup_emitter.model_emitter;
 
 /// The name of the property that stores the tear-off getter on a static
 /// function.
 ///
 /// This property is only used when isolates are used.
@@ skipping 1008 matching lines @@
     }
 
     return js.js.statement(
         '''
         installTearOff(#container, #getterName, #isStatic, #isIntercepted,
                        #requiredParameterCount, #optionalParameterDefaultValues,
                        #callNames, #funsOrNames, #funType)''',
         {
           "container": container,
           "getterName": js.quoteName(method.tearOffName),
-          "isStatic": new js.LiteralBool(method.isStatic),
-          "isIntercepted": new js.LiteralBool(isIntercepted),
+          // 'Truthy' values are ok for `isStatic` and `isIntercepted`.
+          "isStatic": js.number(method.isStatic ? 1 : 0),
+          "isIntercepted": js.number(isIntercepted ? 1 : 0),
           "requiredParameterCount": js.number(requiredParameterCount),
           "optionalParameterDefaultValues": optionalParameterDefaultValues,
           "callNames": callNameArray,
           "funsOrNames": funsOrNamesArray,
           "funType": method.functionType,
         });
   }
 
   /// Wraps the statement in a named function to that it shows up as a unit in

[floitsch, 2017/04/24 13:21:20]

so that

   /// profiles.
   // TODO(sra): Should this be conditional?
   js.Statement wrapPhase(String name, js.Statement statement) {
     return js.js.statement('(function #(){#})();', [name, statement]);
   }
 
   /// Emits the section that installs tear-off getters.
   js.Statement emitInstallTearOffs(Fragment fragment) {
     List<js.Statement> inits = <js.Statement>[];
+    js.Expression temp;
 
     for (Library library in fragment.libraries) {
       for (StaticMethod method in library.statics) {
         // TODO(floitsch): can there be anything else than a StaticDartMethod?
         if (method is StaticDartMethod) {
           if (method.needsTearOff) {
             Holder holder = method.holder;
             inits.add(
                 emitInstallTearOff(new js.VariableUse(holder.name), method));
           }
         }
       }
       for (Class cls in library.classes) {
-        for (InstanceMethod method in cls.methods) {
-          if (method.needsTearOff) {
-            js.Expression container = js.js("#.prototype", classReference(cls));
-            inits.add(emitInstallTearOff(container, method));
+        var methods = cls.methods.where((m) => m.needsTearOff).toList();
+        js.Expression container = js.js("#.prototype", classReference(cls));
+        if (methods.length > 1) {
+          if (temp == null) {
+            inits.add(js.js.statement('var _;'));
+            temp = js.js('_');
           }
+          container = js.js('# = #', [temp, container]);
+        }
+        for (InstanceMethod method in methods) {
+          inits.add(emitInstallTearOff(container, method));
+          container = temp;

[floitsch, 2017/04/24 13:21:20]

Add a comment here, and/or at "container = ...".
The whole thing is a bit too clever without comments.

There are two things that are not obvious:
1. line 1071 is not yet added to the code. That is, at first "use", the temp is
assigned.
2. `container = temp` only matters if there is more than one method, in which
case `temp` is defined. Otherwise, it's just null.

[sra1, 2017/04/25 00:02:19]

Done.

         }
       }
     }
     return wrapPhase('installTearOffs', new js.Block(inits));
   }
 
   /// Emits the constants section.
   js.Statement emitConstants(Fragment fragment) {
     List<js.Statement> assignments = <js.Statement>[];
     for (Constant constant in fragment.constants) {
@@ skipping 357 matching lines @@
     }
     statements.add(js.js.statement("setOrUpdateInterceptorsByTag(#);",
         js.objectLiteral(interceptorsByTag)));
     statements.add(
         js.js.statement("setOrUpdateLeafTags(#);", js.objectLiteral(leafTags)));
     statements.addAll(subclassAssignments);
 
     return wrapPhase('nativeSupport', new js.Block(statements));
   }
 }