dart2js: Avoid inserting redundant HTypeKnown

pkg/compiler/lib/src/ssa/optimize.dart

 // Copyright (c) 2012, 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.
 
 import '../common/codegen.dart' show CodegenRegistry, CodegenWorkItem;
 import '../common/names.dart' show Selectors;
 import '../common/tasks.dart' show CompilerTask;
 import '../compiler.dart' show Compiler;
 import '../constants/constant_system.dart';
 import '../constants/values.dart';
@@ skipping 887 matching lines @@
     return removeIfCheckAlwaysSucceeds(node, node.knownType);
   }
 
   HInstruction removeIfCheckAlwaysSucceeds(HCheck node, TypeMask checkedType) {
     if (checkedType.containsAll(_closedWorld)) return node;
     HInstruction input = node.checkedInput;
     TypeMask inputType = input.instructionType;
     return inputType.isInMask(checkedType, _closedWorld) ? input : node;
   }
 
-  HInstruction removeCheck(HCheck node) => node.checkedInput;
-
   FieldEntity findConcreteFieldForDynamicAccess(
       HInstruction receiver, Selector selector) {
     TypeMask receiverType = receiver.instructionType;
     return _closedWorld.locateSingleField(selector, receiverType);
   }
 
   HInstruction visitFieldGet(HFieldGet node) {
     if (node.isNullCheck) return node;
     var receiver = node.receiver;
 
@@ skipping 1274 matching lines @@
 
   // Update users of [input] that are dominated by [:dominator.first:]
   // to use [TypeKnown] of [input] instead. As the type information depends
   // on the control flow, we mark the inserted [HTypeKnown] nodes as
   // non-movable.
   void insertTypePropagationForDominatedUsers(
       HBasicBlock dominator, HInstruction input, TypeMask convertedType) {
     DominatedUses dominatedUses = DominatedUses.of(input, dominator.first);
     if (dominatedUses.isEmpty) return;
 
+    // Check to avoid adding a duplicate HTypeKnown node.
+    if (dominatedUses.isSingleton) {
+      HInstruction user = dominatedUses.single;
+      if (user is HTypeKnown &&
+          user.isPinned &&
+          user.knownType == convertedType &&
+          user.checkedInput == input) {
+        return;
+      }
+    }
+
     HTypeKnown newInput = new HTypeKnown.pinned(convertedType, input);
     dominator.addBefore(dominator.first, newInput);
     dominatedUses.replaceWith(newInput);
   }
 
   void visitIs(HIs instruction) {
     ResolutionDartType type = instruction.typeExpression;
     if (!instruction.isRawCheck) {
       return;
     } else if (type.isTypedef) {
@@ skipping 647 matching lines @@
 
     keyedValues.forEach((receiver, values) {
       result.keyedValues[receiver] =
           new Map<HInstruction, HInstruction>.from(values);
     });
 
     result.nonEscapingReceivers.addAll(nonEscapingReceivers);
     return result;
   }
 }