dart2js: fast startup cache superclass in inheritance

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

Index: pkg/compiler/lib/src/js_emitter/startup_emitter/fragment_emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/startup_emitter/fragment_emitter.dart b/pkg/compiler/lib/src/js_emitter/startup_emitter/fragment_emitter.dart
index 7dc3001e4f4389f77fd3eeca1f226347818b4d9a..4957d489266759a8a94b353eb02afddc905d0e68 100644
--- a/pkg/compiler/lib/src/js_emitter/startup_emitter/fragment_emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/startup_emitter/fragment_emitter.dart
@@ -822,8 +822,10 @@ class FragmentEmitter {
   /// In this section prototype chains are updated and mixin functions are
   /// copied.
   js.Statement emitInheritance(Fragment fragment) {
-    List<js.Expression> inheritCalls = <js.Expression>[];
-    List<js.Expression> mixinCalls = <js.Expression>[];
+    List<js.Statement> inheritCalls = <js.Statement>[];
+    List<js.Statement> mixinCalls = <js.Statement>[];
+
+    Map<Class, List<Class>> subclasses = <Class, List<Class>>{};
 
     Set<Class> classesInFragment = new Set<Class>();
     for (Library library in fragment.libraries) {
@@ -840,12 +842,7 @@ class FragmentEmitter {
         emitInheritanceForClass(superclass);
       }
 
-      js.Expression superclassReference = (superclass == null)
-          ? new js.LiteralNull()
-          : classReference(superclass);
-
-      inheritCalls.add(
-          js.js('inherit(#, #)', [classReference(cls), superclassReference]));
+      subclasses.putIfAbsent(superclass, () => <Class>[]).add(cls);
 
       emittedClasses.add(cls);
     }
@@ -856,18 +853,37 @@ class FragmentEmitter {
 
         if (cls.isMixinApplication) {
           MixinApplication mixin = cls;
-          mixinCalls.add(js.js('mixin(#, #)',
+          mixinCalls.add(js.js.statement('mixin(#, #)',
               [classReference(cls), classReference(mixin.mixinClass)]));
         }
       }
     }
 
+    js.Expression temp = null;
+    for (Class superclass in subclasses.keys) {
+      List<Class> list = subclasses[superclass];
+      js.Expression superclassReference = (superclass == null)
+          ? new js.LiteralNull()
+          : classReference(superclass);
+      if (list.length == 1) {
+        inheritCalls.add(js.js.statement('inherit(#, #)',
+            [classReference(list.single), superclassReference]));
+      } else {
+        if (temp == null) {
+          inheritCalls.add(js.js.statement('var _ = #', superclassReference));
+          temp = js.js('_');
+        } else {
+          inheritCalls.add(js.js.statement('_ = #', superclassReference));
+        }
+        for (Class cls in list) {
+          inheritCalls
+              .add(js.js.statement('inherit(#, _)', classReference(cls)));
+        }
+      }
+    }
+
     return wrapPhase(
-        'inheritance',
-        js.js.statement('{#; #;}', [
-          inheritCalls.map((e) => new js.ExpressionStatement(e)),
-          mixinCalls.map((e) => new js.ExpressionStatement(e))
-        ]));
+        'inheritance', js.js.statement('{#; #;}', [inheritCalls, mixinCalls]));
   }
 
   /// Emits the setup of method aliases.